feat: Agent Office — AI 에이전트 가상 오피스 (#2)

## Summary
- 2D 픽셀아트 가상 오피스에서 AI 에이전트(Stock, Music)가 실제 작업 수행
- FastAPI + WebSocket 실시간 상태 동기화 + 텔레그램 봇 양방향 알림/승인
- BaseAgent FSM (idle/working/waiting/reporting/break), 서비스 프록시 패턴
- Docker Compose 서비스 (port 18900) + Nginx WebSocket 프록시

## Changes (13 commits)
- Backend scaffold: config, db, models, Dockerfile
- WebSocket manager + Service proxy
- BaseAgent FSM + StockAgent + MusicAgent
- Telegram bot + Scheduler
- FastAPI main (REST + WS endpoints)
- Infrastructure: docker-compose + nginx
- Code review fixes: HTTPException, async polling, input validation

Reviewed-on: #2

agent-office/app/agents/__init__.py

@@ -0,0 +1,17 @@
+from .stock import StockAgent
+from .music import MusicAgent
+
+AGENT_REGISTRY = {}
+
+def init_agents():
+    AGENT_REGISTRY["stock"] = StockAgent()
+    AGENT_REGISTRY["music"] = MusicAgent()
+
+def get_agent(agent_id: str):
+    return AGENT_REGISTRY.get(agent_id)
+
+def get_all_agent_states() -> list:
+    return [
+        {"agent_id": aid, "state": agent.state, "detail": agent.state_detail}
+        for aid, agent in AGENT_REGISTRY.items()
+    ]