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

.github/workflows/ci.yml

@@ -30,5 +30,8 @@ jobs:
     - name: Install all dependencies
       run: uv sync --all-extras
 
+    - name: Lint with ruff
+      run: uv run ruff check nanobot --select F401,F841
+
     - name: Run tests
       run: uv run pytest tests/