CONTRIBUTING.md conflicts with collaborator-only PR restriction

`CONTRIBUTING.md` instructs contributors to fork the repository, push a branch, and submit a pull request, but a fork owned by a non-collaborator cannot open a pull request because the repository UI says “Only collaborators can create PRs” and `gh pr create` fails with a CreatePullRequest permission error; consequently, the documented contribution flow cannot be completed by a non-collaborator.

打开 CONTRIBUTING.md，并根据协作者限制和报告的 `gh pr create` 权限错误，检查有关 fork、branch 和 pull request 的说明。更新文档中的贡献流程，使非协作者能够遵循有效流程，然后验证这些说明不再承诺不可用的操作。

由索引模型根据 Issue 内容生成。