CI: Flag issue mentions that are not linked in the Development field

[adamsilverstein]

What?

Closes #77397

Adds a new advisory CI workflow, check-linked-issues.yml, that scans every open PR's description for issue references (#1234, GH-1234, or full issue URLs) and compares them against the PR's Development relationship (GraphQL closingIssuesReferences). If any referenced issue isn't linked, the workflow posts or updates a sticky comment explaining how to link it.

Why?

Based on Amy's feedback: anything included in a release should be linked (not just mentioned) in the PR description so that it shows up in the Development section on the right sidebar of the related ticket. Today, tracking tickets like the per-release editor tasks ticket rely on release leads manually following every comment and every PR mentioned in every tracker to figure out what belongs in the release — and things still slip through. Surfacing the relationship in the sidebar lets release leads and release-note writers find every PR attached to a ticket at a glance.

The check is intentionally non-invasive:

• It does not edit the PR body.
• It does not fail the PR — it's advisory.
• It only comments when unlinked mentions are detected; it clears its own comment once they're linked.
• Informational mentions (e.g. Follow up to Confirm navigation when user has unsaved changes #1234, See Confirm navigation when user has unsaved changes #1234) can be left as-is if intentional.

A more aggressive variant (auto-appending a bot-managed Closes #N block to the PR body) was considered but not chosen, because closing keywords also auto-close the referenced issue on merge — which is wrong for informational references. A future iteration can layer automatic linking on top of this check if desired.

How?

• New workflow: .github/workflows/check-linked-issues.yml.
• Runs on pull_request_target (opened, edited, reopened, synchronize, ready_for_review) so it reacts to description edits.
• Uses actions/github-script to:
  1. Strip HTML comments from the body (avoids matching the Closes <!-- #ISSUE-NUMBER --> placeholder in the PR template).
  2. Extract referenced numbers from bare #N, GH-N, and in-repo issue/pull URLs.
  3. Query closingIssuesReferences to determine which are already linked via the Development field.
  4. Verify each remaining number resolves to an issue (not a PR and not 404) to filter cross-repo and stale references.
  5. Post or update a single sticky comment (marked with an HTML-comment tag) listing unlinked references; delete the comment once everything is linked.
• Uses the default GITHUB_TOKEN with scoped pull-requests: write permission. No additional secrets required.
• Gated to WordPress/gutenberg and open PRs only.
• The script itself is annotated with step-by-step block comments (Step 1–10) to make the flow easy to follow for future maintainers.

Testing Instructions

1. Open or edit a test PR that mentions an issue in the body without a closing keyword (e.g. See #100).
2. Confirm the workflow runs and posts a comment listing the unlinked issue.
3. Update the PR body to use Closes #100 (or manually link the issue from the Development sidebar).
4. Re-trigger the workflow (edit the body or push) and confirm the comment is removed.
5. Verify a PR description with no issue mentions produces no comment.
6. Verify a PR description that only mentions PR numbers (not issue numbers) produces no comment.

Follow-ups

• If this approach works well, consider tightening to a required check (exit_type: failure) after a grace period.
• Could be extended to enforce the relationship for PRs labeled with a release milestone only.
• Optional: layer in automatic linking for references that clearly use closing verbs in prose (e.g. fixes issue Merge Single TinyMCE Instance #123) but aren't formatted correctly.

[github-actions]

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

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[github-actions]

Flaky tests detected in 9d4fe16.
Some tests passed with failed attempts. The failures may not be related to this commit but are still reported for visibility. See the documentation for more information.

🔍 Workflow run URL: https://github.com/WordPress/gutenberg/actions/runs/24481862957
📝 Reported issues:

• [Flaky Test] Navigates from items list to preview via TAB, and vice versa #62434 in /test/e2e/specs/site-editor/dataviews-list-layout-keyboard.spec.js
• [Flaky Test] post title and content are editable and blocks can be inserted #76524 in /test/e2e/specs/editor/various/post-content-focus-mode.spec.js
• [Flaky Test] should multi-select block with text selection and a block without text selection #59579 in /test/e2e/specs/editor/various/multi-block-selection.spec.js
• [Flaky Test] should show correct homepage actions based on current homepage or posts page #77385 in /test/e2e/specs/site-editor/homepage-settings.spec.js