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_preprocessors.py

@@ -12,7 +12,6 @@ from __future__ import annotations
 import re
 from pathlib import Path
 
-import pytest
 import yaml
 
 from app.core.preprocessors import detect_content_type, preprocess