ci: add ruff lint check for unused imports and variables

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/feishu.py

@@ -9,7 +9,6 @@ import time
 import uuid
 from collections import OrderedDict
 from dataclasses import dataclass
-from pathlib import Path
 from typing import Any, Literal
 
 from loguru import logger