feat: Add merge conflict detection and agent resolution in initiative review

Pre-merge mergeability check via `git merge-tree --write-tree` (dry-run, no
side effects). When conflicts exist the "Merge & Push" button is disabled and
a ConflictResolutionPanel shows conflict files with options to resolve manually
or spawn a conflict-resolution agent. Agent questions appear inline via
QuestionForm; on completion the mergeability re-checks automatically.

New server-side: MergeabilityResult type, BranchManager.checkMergeability,
conflict-resolution prompt, checkInitiativeMergeability query,
spawnConflictResolutionAgent mutation, getActiveConflictAgent query.

New frontend: useConflictAgent hook, ConflictResolutionPanel component,
mergeability badge + panel integration in InitiativeReview.

apps/server/agent/prompts/conflict-resolution.ts

@@ -0,0 +1,74 @@
+/**
+ * Conflict resolution prompt — spawned when initiative branch has merge conflicts
+ * with the target branch.
+ */
+
+import {
+  SIGNAL_FORMAT,
+  SESSION_STARTUP,
+  GIT_WORKFLOW,
+  CONTEXT_MANAGEMENT,
+} from './shared.js';
+
+export function buildConflictResolutionPrompt(
+  sourceBranch: string,
+  targetBranch: string,
+  conflicts: string[],
+): string {
+  const conflictList = conflicts.map(f => `- \`${f}\``).join('\n');
+
+  return `<role>
+You are a Conflict Resolution agent. Your job is to merge \`${targetBranch}\` into \`${sourceBranch}\` and resolve all merge conflicts so the initiative branch is up to date with the target branch.
+</role>
+
+<conflict_details>
+**Source branch (initiative):** \`${sourceBranch}\`
+**Target branch (default):** \`${targetBranch}\`
+
+**Conflicting files:**
+${conflictList}
+</conflict_details>
+${SIGNAL_FORMAT}
+${SESSION_STARTUP}
+
+<resolution_protocol>
+Follow these steps in order:
+
+1. **Inspect divergence**: Run \`git log --oneline ${targetBranch}..${sourceBranch}\` and \`git log --oneline ${sourceBranch}..${targetBranch}\` to understand what each side changed.
+
+2. **Review conflicting files**: For each conflicting file, read both versions:
+   - \`git show ${sourceBranch}:<file>\`
+   - \`git show ${targetBranch}:<file>\`
+
+3. **Merge**: Run \`git merge ${targetBranch} --no-edit\`. This will produce conflict markers.
+
+4. **Resolve each file**: For each conflicting file:
+   - Read the file to see conflict markers (\`<<<<<<<\`, \`=======\`, \`>>>>>>>\`)
+   - Understand both sides' intent from step 1-2
+   - Choose the correct resolution — keep both changes when they don't overlap, prefer the more complete version when they do
+   - If you genuinely cannot determine the correct resolution, signal "questions" explaining the ambiguity
+
+5. **Verify**: Run \`git diff --check\` to confirm no conflict markers remain. Run the test suite to confirm nothing is broken.
+
+6. **Commit**: Stage resolved files with \`git add <file>\` (never \`git add .\`), then \`git commit --no-edit\` to complete the merge commit.
+
+7. **Signal done**: Write signal.json with status "done".
+</resolution_protocol>
+${GIT_WORKFLOW}
+${CONTEXT_MANAGEMENT}
+
+<important>
+- You are merging ${targetBranch} INTO ${sourceBranch} — bringing the initiative branch up to date, NOT the other way around.
+- Do NOT force-push or rebase. A merge commit is the correct approach.
+- If tests fail after resolution, fix the code — don't skip tests.
+- If a conflict is genuinely ambiguous (e.g., both sides rewrote the same function differently), signal "questions" with the specific ambiguity and your proposed resolution.
+</important>`;
+}
+
+export function buildConflictResolutionDescription(
+  sourceBranch: string,
+  targetBranch: string,
+  conflicts: string[],
+): string {
+  return `Resolve ${conflicts.length} merge conflict(s) between ${sourceBranch} and ${targetBranch}: ${conflicts.join(', ')}`;
+}

apps/server/agent/prompts/index.ts

@@ -15,3 +15,4 @@ export { buildChatPrompt } from './chat.js';
 export type { ChatHistoryEntry } from './chat.js';
 export { buildWorkspaceLayout } from './workspace.js';
 export { buildPreviewInstructions } from './preview.js';
+export { buildConflictResolutionPrompt, buildConflictResolutionDescription } from './conflict-resolution.js';

apps/server/execution/orchestrator.test.ts

@@ -48,6 +48,7 @@ function createMocks() {
     diffCommit: vi.fn().mockResolvedValue(''),
     getMergeBase: vi.fn().mockResolvedValue('abc123'),
     pushBranch: vi.fn(),
+    checkMergeability: vi.fn().mockResolvedValue({ mergeable: true }),
   };
 
   const phaseRepository = {

apps/server/git/branch-manager.ts

@@ -6,7 +6,7 @@
  * a worktree to be checked out.
  */
 
-import type { MergeResult, BranchCommit } from './types.js';
+import type { MergeResult, MergeabilityResult, BranchCommit } from './types.js';
 
 export interface BranchManager {
   /**
@@ -68,4 +68,11 @@ export interface BranchManager {
    * Defaults to 'origin' if no remote specified.
    */
   pushBranch(repoPath: string, branch: string, remote?: string): Promise<void>;
+
+  /**
+   * Dry-run merge check — determines if sourceBranch can be cleanly merged
+   * into targetBranch without actually performing the merge.
+   * Uses `git merge-tree --write-tree` (git 2.38+).
+   */
+  checkMergeability(repoPath: string, sourceBranch: string, targetBranch: string): Promise<MergeabilityResult>;
 }

apps/server/git/index.ts

@@ -13,7 +13,7 @@
 export type { WorktreeManager } from './types.js';
 
 // Domain types
-export type { Worktree, WorktreeDiff, MergeResult } from './types.js';
+export type { Worktree, WorktreeDiff, MergeResult, MergeabilityResult } from './types.js';
 
 // Adapters
 export { SimpleGitWorktreeManager } from './manager.js';

apps/server/git/simple-git-branch-manager.ts

@@ -11,7 +11,7 @@ import { mkdtempSync, rmSync } from 'node:fs';
 import { tmpdir } from 'node:os';
 import { simpleGit } from 'simple-git';
 import type { BranchManager } from './branch-manager.js';
-import type { MergeResult, BranchCommit } from './types.js';
+import type { MergeResult, MergeabilityResult, BranchCommit } from './types.js';
 import { createModuleLogger } from '../logger/index.js';
 
 const log = createModuleLogger('branch-manager');
@@ -164,4 +164,41 @@ export class SimpleGitBranchManager implements BranchManager {
     await git.push(remote, branch);
     log.info({ repoPath, branch, remote }, 'branch pushed to remote');
   }
+
+  async checkMergeability(repoPath: string, sourceBranch: string, targetBranch: string): Promise<MergeabilityResult> {
+    const git = simpleGit(repoPath);
+
+    try {
+      // git merge-tree --write-tree merges source INTO target virtually.
+      // Exit 0 = clean merge, non-zero = conflicts.
+      await git.raw(['merge-tree', '--write-tree', targetBranch, sourceBranch]);
+      log.debug({ repoPath, sourceBranch, targetBranch }, 'merge-tree check: clean');
+      return { mergeable: true };
+    } catch (err) {
+      const stderr = err instanceof Error ? err.message : String(err);
+
+      // Parse conflict file names from "CONFLICT (content): Merge conflict in <path>"
+      const conflictPattern = /CONFLICT \([^)]+\): (?:Merge conflict in|.* -> )(.+)/g;
+      const conflicts: string[] = [];
+      let match: RegExpExecArray | null;
+      while ((match = conflictPattern.exec(stderr)) !== null) {
+        conflicts.push(match[1].trim());
+      }
+
+      if (conflicts.length > 0) {
+        log.debug({ repoPath, sourceBranch, targetBranch, conflicts }, 'merge-tree check: conflicts');
+        return { mergeable: false, conflicts };
+      }
+
+      // If we couldn't parse conflicts but the command failed, it's still a conflict
+      // (could be add/add, rename conflicts, etc.)
+      if (stderr.includes('CONFLICT')) {
+        log.debug({ repoPath, sourceBranch, targetBranch }, 'merge-tree check: unparsed conflicts');
+        return { mergeable: false, conflicts: ['(unable to parse conflict details)'] };
+      }
+
+      // Genuine error (not a conflict)
+      throw err;
+    }
+  }
 }

apps/server/git/types.ts

@@ -58,6 +58,19 @@ export interface MergeResult {
   message: string;
 }
 
+// =============================================================================
+// Mergeability Check
+// =============================================================================
+
+/**
+ * Result of a dry-run merge check.
+ * No side effects — only tells you whether the merge would succeed.
+ */
+export interface MergeabilityResult {
+  mergeable: boolean;
+  conflicts?: string[];
+}
+
 // =============================================================================
 // Branch Commit Info
 // =============================================================================

apps/server/trpc/routers/agent.ts

@@ -184,6 +184,27 @@ export function agentProcedures(publicProcedure: ProcedureBuilder) {
         return candidates[0] ?? null;
       }),
 
+    getActiveConflictAgent: publicProcedure
+      .input(z.object({ initiativeId: z.string().min(1) }))
+      .query(async ({ ctx, input }): Promise<AgentInfo | null> => {
+        const agentManager = requireAgentManager(ctx);
+        const allAgents = await agentManager.list();
+        const candidates = allAgents
+          .filter(
+            (a) =>
+              a.mode === 'execute' &&
+              a.initiativeId === input.initiativeId &&
+              a.name?.startsWith('conflict-') &&
+              ['running', 'waiting_for_input', 'idle', 'crashed'].includes(a.status) &&
+              !a.userDismissedAt,
+          )
+          .sort(
+            (a, b) =>
+              new Date(b.createdAt).getTime() - new Date(a.createdAt).getTime(),
+          );
+        return candidates[0] ?? null;
+      }),
+
     getAgentOutput: publicProcedure
       .input(agentIdentifierSchema)
       .query(async ({ ctx, input }): Promise<string> => {

apps/server/trpc/routers/initiative.ts

@@ -7,7 +7,7 @@ import { z } from 'zod';
 import type { ProcedureBuilder } from '../trpc.js';
 import { requireAgentManager, requireInitiativeRepository, requireProjectRepository, requireTaskRepository, requireBranchManager, requireExecutionOrchestrator } from './_helpers.js';
 import { deriveInitiativeActivity } from './initiative-activity.js';
-import { buildRefinePrompt } from '../../agent/prompts/index.js';
+import { buildRefinePrompt, buildConflictResolutionPrompt, buildConflictResolutionDescription } from '../../agent/prompts/index.js';
 import type { PageForSerialization } from '../../agent/content-serializer.js';
 import { ensureProjectClone } from '../../git/project-clones.js';
 
@@ -349,5 +349,128 @@ export function initiativeProcedures(publicProcedure: ProcedureBuilder) {
         );
         return { success: true, taskId: result.taskId };
       }),
+
+    checkInitiativeMergeability: publicProcedure
+      .input(z.object({ initiativeId: z.string().min(1) }))
+      .query(async ({ ctx, input }) => {
+        const initiativeRepo = requireInitiativeRepository(ctx);
+        const projectRepo = requireProjectRepository(ctx);
+        const branchManager = requireBranchManager(ctx);
+
+        const initiative = await initiativeRepo.findById(input.initiativeId);
+        if (!initiative) {
+          throw new TRPCError({ code: 'NOT_FOUND', message: `Initiative '${input.initiativeId}' not found` });
+        }
+        if (!initiative.branch) {
+          throw new TRPCError({ code: 'BAD_REQUEST', message: 'Initiative has no branch configured' });
+        }
+
+        const projects = await projectRepo.findProjectsByInitiativeId(input.initiativeId);
+        const allConflicts: string[] = [];
+        let mergeable = true;
+
+        for (const project of projects) {
+          const clonePath = await ensureProjectClone(project, ctx.workspaceRoot!);
+          const result = await branchManager.checkMergeability(clonePath, initiative.branch, project.defaultBranch);
+          if (!result.mergeable) {
+            mergeable = false;
+            if (result.conflicts) allConflicts.push(...result.conflicts);
+          }
+        }
+
+        return {
+          mergeable,
+          conflictFiles: allConflicts,
+          targetBranch: projects[0]?.defaultBranch ?? 'main',
+        };
+      }),
+
+    spawnConflictResolutionAgent: publicProcedure
+      .input(z.object({
+        initiativeId: z.string().min(1),
+        provider: z.string().optional(),
+      }))
+      .mutation(async ({ ctx, input }) => {
+        const agentManager = requireAgentManager(ctx);
+        const initiativeRepo = requireInitiativeRepository(ctx);
+        const projectRepo = requireProjectRepository(ctx);
+        const taskRepo = requireTaskRepository(ctx);
+        const branchManager = requireBranchManager(ctx);
+
+        const initiative = await initiativeRepo.findById(input.initiativeId);
+        if (!initiative) {
+          throw new TRPCError({ code: 'NOT_FOUND', message: `Initiative '${input.initiativeId}' not found` });
+        }
+        if (!initiative.branch) {
+          throw new TRPCError({ code: 'BAD_REQUEST', message: 'Initiative has no branch configured' });
+        }
+
+        const projects = await projectRepo.findProjectsByInitiativeId(input.initiativeId);
+        if (projects.length === 0) {
+          throw new TRPCError({ code: 'BAD_REQUEST', message: 'Initiative has no linked projects' });
+        }
+
+        // Auto-dismiss stale conflict agents
+        const allAgents = await agentManager.list();
+        const staleAgents = allAgents.filter(
+          (a) =>
+            a.mode === 'execute' &&
+            a.initiativeId === input.initiativeId &&
+            a.name?.startsWith('conflict-') &&
+            ['crashed', 'idle'].includes(a.status) &&
+            !a.userDismissedAt,
+        );
+        for (const stale of staleAgents) {
+          await agentManager.dismiss(stale.id);
+        }
+
+        // Reject if active conflict agent already running
+        const activeConflictAgents = allAgents.filter(
+          (a) =>
+            a.mode === 'execute' &&
+            a.initiativeId === input.initiativeId &&
+            a.name?.startsWith('conflict-') &&
+            ['running', 'waiting_for_input'].includes(a.status),
+        );
+        if (activeConflictAgents.length > 0) {
+          throw new TRPCError({
+            code: 'CONFLICT',
+            message: 'A conflict resolution agent is already running for this initiative',
+          });
+        }
+
+        // Re-check mergeability to get current conflict list
+        const project = projects[0];
+        const clonePath = await ensureProjectClone(project, ctx.workspaceRoot!);
+        const mergeCheck = await branchManager.checkMergeability(clonePath, initiative.branch, project.defaultBranch);
+        if (mergeCheck.mergeable) {
+          throw new TRPCError({ code: 'BAD_REQUEST', message: 'No merge conflicts detected — merge is clean' });
+        }
+
+        const conflicts = mergeCheck.conflicts ?? [];
+        const targetBranch = project.defaultBranch;
+
+        // Create task
+        const task = await taskRepo.create({
+          initiativeId: input.initiativeId,
+          name: `Resolve conflicts: ${initiative.name}`,
+          description: buildConflictResolutionDescription(initiative.branch, targetBranch, conflicts),
+          category: 'merge',
+          status: 'in_progress',
+        });
+
+        // Spawn agent
+        const prompt = buildConflictResolutionPrompt(initiative.branch, targetBranch, conflicts);
+        return agentManager.spawn({
+          name: `conflict-${Date.now()}`,
+          taskId: task.id,
+          prompt,
+          mode: 'execute',
+          provider: input.provider,
+          initiativeId: input.initiativeId,
+          baseBranch: targetBranch,
+          branchName: initiative.branch,
+        });
+      }),
   };
 }