feat: errand review & request changes

Add errand.requestChanges procedure that re-spawns an agent in the
existing worktree with user feedback. Replace raw <pre> diff blocks
with syntax-highlighted ErrandDiffView using FileCard components.
Add Output/Changes tabs to the active errand view.

apps/server/trpc/routers/errand.ts

@@ -1,7 +1,7 @@
 /**
  * Errand Router
  *
- * All 9 errand procedures: create, list, get, diff, complete, merge, delete, sendMessage, abandon.
+ * All 10 errand procedures: create, list, get, diff, complete, merge, delete, sendMessage, abandon, requestChanges.
  * Errands are small isolated changes that spawn a dedicated agent in a git worktree.
  */
 
@@ -17,8 +17,9 @@ import {
   requireBranchManager,
 } from './_helpers.js';
 import { writeErrandManifest } from '../../agent/file-io.js';
-import { buildErrandPrompt } from '../../agent/prompts/index.js';
+import { buildErrandPrompt, buildErrandRevisionPrompt } from '../../agent/prompts/index.js';
 import { join } from 'node:path';
+import { existsSync, rmSync } from 'node:fs';
 import { SimpleGitWorktreeManager } from '../../git/manager.js';
 import { ensureProjectClone, getProjectCloneDir } from '../../git/project-clones.js';
 import type { TRPCContext } from '../context.js';
@@ -441,6 +442,96 @@ export function errandProcedures(publicProcedure: ProcedureBuilder) {
           const updated = await repo.update(input.id, { status: 'abandoned' });
           return updated;
         }),
+
+      // -----------------------------------------------------------------------
+      // errand.requestChanges
+      // -----------------------------------------------------------------------
+      requestChanges: publicProcedure
+        .input(z.object({
+          id: z.string().min(1),
+          feedback: z.string().min(1),
+        }))
+        .mutation(async ({ ctx, input }) => {
+          const repo = requireErrandRepository(ctx);
+          const errand = await repo.findById(input.id);
+          if (!errand) {
+            throw new TRPCError({ code: 'NOT_FOUND', message: 'Errand not found' });
+          }
+
+          if (errand.status !== 'pending_review' && errand.status !== 'conflict') {
+            throw new TRPCError({
+              code: 'BAD_REQUEST',
+              message: `Cannot request changes on an errand with status '${errand.status}'`,
+            });
+          }
+
+          if (!errand.projectId) {
+            throw new TRPCError({ code: 'NOT_FOUND', message: 'Errand has no project' });
+          }
+          const project = await requireProjectRepository(ctx).findById(errand.projectId);
+          if (!project) {
+            throw new TRPCError({ code: 'NOT_FOUND', message: 'Project not found' });
+          }
+
+          // Resolve clone path and verify worktree still exists
+          const clonePath = await resolveClonePath(project, ctx);
+          const worktreeManager = new SimpleGitWorktreeManager(clonePath);
+          let worktree;
+          try {
+            worktree = await worktreeManager.get(errand.id);
+          } catch {
+            throw new TRPCError({
+              code: 'BAD_REQUEST',
+              message: 'Worktree no longer exists — cannot request changes. Delete and re-create the errand.',
+            });
+          }
+          if (!worktree) {
+            throw new TRPCError({
+              code: 'BAD_REQUEST',
+              message: 'Worktree no longer exists — cannot request changes. Delete and re-create the errand.',
+            });
+          }
+
+          // Clean up stale signal.json to prevent false completion detection
+          const signalPath = join(worktree.path, '.cw', 'output', 'signal.json');
+          if (existsSync(signalPath)) {
+            rmSync(signalPath);
+          }
+
+          // Build revision prompt and spawn new agent in existing worktree
+          const prompt = buildErrandRevisionPrompt(errand.description, input.feedback);
+          const agentManager = requireAgentManager(ctx);
+          let agent;
+          try {
+            agent = await agentManager.spawn({
+              prompt,
+              mode: 'errand',
+              cwd: worktree.path,
+              provider: undefined,
+            });
+          } catch (err) {
+            throw new TRPCError({
+              code: 'INTERNAL_SERVER_ERROR',
+              message: err instanceof Error ? err.message : String(err),
+            });
+          }
+
+          // Update manifest files
+          await writeErrandManifest({
+            agentWorkdir: worktree.path,
+            errandId: errand.id,
+            description: errand.description,
+            branch: errand.branch,
+            projectName: project.name,
+            agentId: agent.id,
+            agentName: agent.name,
+          });
+
+          // Transition back to active with new agent
+          await repo.update(errand.id, { status: 'active', agentId: agent.id });
+
+          return { id: errand.id, agentId: agent.id };
+        }),
     }),
   };
 }