fix: Show completed phase diffs in review tab

Completed phases showed "No phases pending review" because:
1. Frontend filtered only pending_review phases
2. Server rejected non-pending_review phases
3. After merge, three-dot diff returned empty (merge base moved)

Fix: store pre-merge merge base hash on phase, use it to reconstruct
diffs for completed phases. Frontend now shows both pending_review and
completed phases with read-only mode (Merged badge) for completed ones.

apps/server/agent/file-io.test.ts

@@ -68,6 +68,7 @@ describe('writeInputFiles', () => {
       name: 'Phase One',
       content: 'First phase',
       status: 'pending',
+      mergeBase: null,
       createdAt: new Date(),
       updatedAt: new Date(),
     } as Phase;