Build Tools: Add workflow to label original PRs on manual cherry-pick…

[Sahitya3105]

What?

Closes #76579

Adds a new GitHub Actions workflow (.github/workflows/label-manual-cherry-pick.yml) that automatically detects manual cherry-picks when a PR is merged to a wp/* or release/* branch and labels the original trunk PR with Backported to WP Core.

Why?

When the automatic cherry-pick workflow fails due to merge conflicts, contributors must manually git cherry-pick and create a PR to wp/* or release/* branches. However, they often forget to add the Backported to WP Core label to the original trunk PR, causing it to be missed in programmatically generated lists.

How?

The workflow triggers on pull_request_target → closed (merged) events targeting wp/** or release/** branches and uses four detection strategies to find the original PR number:

1. Explicit cherry-pick references in PR title/body — e.g., cherry-pick of #12345
2. GitHub PR URL references — e.g., cherry-picked from https://github.com/WordPress/gutenberg/pull/12345
3. Git's automatic annotation — resolves (cherry picked from commit <SHA>) from the merge commit to the original trunk PR via the GitHub API
4. PR number in commit messages — extracts (#12345) patterns from squashed commit messages

After detection, it validates the original PR targets trunk, skips already-labeled PRs for idempotency, and applies the Backported to WP Core label.

Testing Instructions

This workflow only runs in WordPress/gutenberg when a PR is merged to wp/** or release/** branches. To verify:

1. Review the workflow YAML for correctness.
2. Confirm the regex patterns match expected cherry-pick reference formats (e.g., cherry-pick of #12345, cherry-picked from #12345).
3. Verify the GitHub API calls are correct (git.getCommit, repos.listPullRequestsAssociatedWithCommit, pulls.get, issues.addLabels).
4. Confirm the workflow follows project conventions (pinned action SHAs, permission model, runner version).

Testing Instructions for Keyboard

N/A — This is a CI workflow change with no UI modifications.

Screenshots or screencast

N/A — No UI changes.

Use of AI Tools

This PR was authored with the assistance of an AI coding assistant (Gemini) for code generation and review. All code has been reviewed and validated by the contributor.

[github-actions]

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Unlinked Accounts

The following contributors have not linked their GitHub and WordPress.org accounts: @Sahitya3105, @JiwaniZakir, @Kunal152000.

Contributors, please read how to link your accounts to ensure your work is properly credited in WordPress releases.

If you're merging code through a pull request on GitHub, copy and paste the following into the bottom of the merge commit message.

Unlinked contributors: Sahitya3105, JiwaniZakir, Kunal152000.

Co-authored-by: desrosj <desrosj@git.wordpress.org>

To understand the WordPress project's expectations around crediting contributors, please review the Contributor Attribution page in the Core Handbook.

[github-actions]

👋 Thanks for your first Pull Request and for helping build the future of Gutenberg and WordPress, @Sahitya3105! In case you missed it, we'd love to have you join us in our Slack community.

If you want to learn more about WordPress development in general, check out the Core Handbook full of helpful information.

[Sahitya3105]

Could a maintainer please add the [Type] Build Tooling label? This PR adds a new GitHub Actions workflow. Thank you!

[Sahitya3105]

Hi! Just a gentle follow-up on this PR. All checks seem to be passing now—would appreciate a review when convenient. Thanks!