Add CI step to detect unused imports (F401) and unused variables (F841)
with ruff. Clean up existing violations:
- Remove unused Consolidator import in agent/__init__.py
- Remove unused re import in agent/loop.py
- Remove unused Path import in channels/feishu.py
- Remove unused ContentRepositoryConfigError import in channels/matrix.py
- Remove unused field and CommandHandler imports in channels/telegram.py
- Remove unused exception variable in channels/weixin.py
- nanobot/channels/matrix.py: Add keyword-only parameters restrict_to_workspace/workspace to MatrixChannel.__init__ and assign them to _restrict_to_workspace/_workspace with proper type conversion and path resolution
- tests/test_commands.py: Add _strip_ansi() function to remove ANSI escape codes, use regex assertions for --workspace/--config parameters to allow 1 or 2 dashes