Support backport labels for release-branch CI and post-merge backports

[Yicong-Huang]

Task Summary

Support a backport <release-branch> label workflow that runs CI against the labeled release branch and performs the agreed post-merge backport automation.

Priority

P1 – High

Task Type

• Code Implementation
• Documentation
• Refactor / Cleanup
• Testing / QA
• DevOps / Deployment

Notes

• Detect backport labels on PRs.
• Run CI by applying the PR changes against the labeled release branch.
• After the PR is merged into main, create or push the corresponding backport change for the release branch according to the agreed ASF-compliant workflow.
• Document the contributor and committer workflow for release-branch backports.