feat(agent-office): notification broadcast + telegram tracking + activity feed API

- Add WebSocket notification messages for task_assigned/task_completed
- Structure telegram send_message return value with ok/message_id
- Track telegram delivery status in task result_data
- Add test_telegram command to stock agent
- Add GET /api/agent-office/activity unified feed endpoint

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

agent-office/app/agents/base.py

@@ -40,6 +40,14 @@ class BaseAgent:
 
         if self._ws_manager:
             await self._ws_manager.send_agent_state(self.agent_id, new_state, detail, task_id)
+            if new_state == "working" and old != "working":
+                await self._ws_manager.send_notification(
+                    self.agent_id, "task_assigned", task_id, detail or "새 작업 시작"
+                )
+            elif new_state == "idle" and old in ("working", "reporting"):
+                await self._ws_manager.send_notification(
+                    self.agent_id, "task_completed", task_id, detail or "작업 완료"
+                )
             if new_state == "break":
                 await self._ws_manager.send_agent_move(self.agent_id, "break_room")
             elif old == "break" and new_state == "idle":