refactor(test): consolidate FakeApp and add sys.modules isolation utility

- Extract tests/utils/import_isolation.py with isolated_sys_modules context manager
- Extend tests/factories/app.py FakeApp with handler-specific attributes
- Refactor test_chat_handler.py to use centralized FakeApp and cached imports
- Refactor test_command_handler.py with mock_execute_factory fixture
- Refactor test_smoke.py to move import-time sys.modules manipulation into fixture
- Add SQLite migration integration tests (G-002)
- Add HTTP API smoke integration tests (G-005)
- Update CI workflow to call pytest for SQLite migrations (G-004)

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/integration/__init__.py

@@ -0,0 +1,6 @@
+"""
+Integration tests package.
+
+These tests validate real system behavior with actual database/network resources.
+Run with: uv run pytest tests/integration/ -m "not slow" -q
+"""