feat: auto-notify user on paired task completion with @mention

- done → ✅ 작업 완료
- escalated → ⚠️ 자동 해결 불가
- arbiter_escalated → ⚠️ 중재자: 사람 개입 필요
- stopped → no notification (user initiated)

src/message-agent-executor.test.ts

@@ -48,7 +48,11 @@ vi.mock('./platform-prompts.js', () => ({
 vi.mock('./db.js', () => ({
   createServiceHandoff: vi.fn(),
   getAllTasks: vi.fn(() => []),
+  getLastHumanMessageSender: vi.fn(() => null),
   getLatestOpenPairedTaskForChat: vi.fn(() => undefined),
+  getLatestTurnNumber: vi.fn(() => 0),
+  getPairedTaskById: vi.fn(() => undefined),
+  insertPairedTurnOutput: vi.fn(),
 }));
 
 vi.mock('./service-routing.js', () => ({