Audit flake8 CI errors vs. warnings

#1 mentions that the flake8 in `workflows/lint.yml` may be overly permissive. This could result in bad code getting in.

- [ ] evaluate error on **F401** for unused imports
- [ ] evaluate error on **F841** for local variable assigned but unused

Note that pre-commit config already prohibits both of these.

先閱讀 workflows/lint.yml 和現有的 pre-commit 設定。比較 flake8 如何處理 F401 和 F841 與 pre-commit 已禁止的檢查，然後判斷是否需要調整 CI 設定。完成的條件是：這兩項檢查都有明確且有文件記錄的 CI 決策，且相關的 lint 設定一致。

由索引模型根據 Issue 內容生成。