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/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 = {