refactor(chatkit): migrate to official OpenAI ChatKit SDK

Replace legacy manual ChatKit protocol implementation with official SDK:

- Delete backend/src/api/chatkit.py (858 lines of manual implementation)
- Add chatkit_official.py using official ChatKitServer class
- Add ChatKitDatabaseStore implementing official Store interface
- Add LifeStepsAIChatKitServer extending ChatKitServer
- Update all test imports to use chatkit_official
- Use stream_agent_response() and simple_to_agent_input() helpers

All Phase III requirements remain validated:
- OpenAI Agents SDK, MCP SDK, ChatKit SDK properly integrated
- 5 MCP tools (add/list/complete/delete/update) working
- Stateless architecture maintained

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

Author: DanielHashmi

backend/main.py

@@ -15,7 +15,8 @@
 from src.api.auth import router as auth_router
 from src.api.tasks import router as tasks_router
 from src.api.profile import router as profile_router
-from src.api.chatkit import router as chatkit_router
+# Official ChatKit integration (ChatKitServer with stream_agent_response)
+from src.api.chatkit_official import router as chatkit_router
 from src.api.reminders import router as reminders_router
 from src.api.notification_settings import router as notification_settings_router
 from src.services.notification_service import notification_polling_loop

backend/src/api/__init__.py

@@ -1,6 +1,6 @@
 # API package
 from .auth import router as auth_router
-from .chatkit import router as chatkit_router
+from .chatkit_official import router as chatkit_router  # Official ChatKit protocol
 from .reminders import router as reminders_router
 from .notification_settings import router as notification_settings_router
 from .jobs import router as jobs_router