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/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';