feat(i18n): inject user language into AI agent system prompts

- Add _language_instruction() to deep_agent.py, reads language from core memory
- Append language directive to all 4 run_* functions (task/project/checkpoint/note)
- Minor fixes: alembic env, route imports, test cleanup

app/core/agent_runner.py

@@ -30,7 +30,6 @@ import asyncio
 import json
 import logging
 import os
-import uuid
 from datetime import datetime, timedelta, timezone
 from typing import Any