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/weixin.py

@@ -484,7 +484,7 @@ class WeixinChannel(BaseChannel):
             except httpx.TimeoutException:
                 # Normal for long-poll, just retry
                 continue
-            except Exception as e:
+            except Exception:
                 if not self._running:
                     break
                 consecutive_failures += 1