ci: integrate CLA Assistant workflow

[laipz8200]

Important

1. Make sure you have read our contribution guidelines
2. Use a Conventional Commits title for this pull request, and mark breaking changes with !
3. Keep the change focused and reviewable, and link any related issue when applicable

Summary

• add a CLA Assistant GitHub Actions workflow for pull requests and CLA recheck comments
• add CLA.md and document the contributor signing flow in CONTRIBUTING.md and README.md
• update the pull request checklist to cover CLA signing when prompted
• configure signature storage on the dedicated cla-signatures branch so the protected default branch does not need workflow writes

Checklist

• This pull request title follows Conventional Commits, and any breaking change is marked with !
• I ran make pre before pushing this branch
• If CLA Assistant prompted me, I signed CLA.md in the pull request conversation
• I added or updated tests for behavior changes, or this change does not require tests
• I updated the relevant repository documentation when needed
• This change is focused and reviewable