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

tests/test_agent_runner_v2.py

@@ -40,7 +40,6 @@ from app.core.agent_runner import (
     _format_projects,
     _get_extraction_rules,
     _get_no_match_behavior,
-    _is_overdue,
     run_local_agent,
 )
 from app.core.device_manager import DeviceConnectionManager