chore: add script to audit changelog labels on merged PRs

[paulbalandan]

Description
Follow-up to #10390. The changelog quality depends on PRs being labeled before the release notes are generated, but missing labels currently only surface at release time via the catch-all "Others" category. This PR adds admin/check-pr-labels.php (requires the authenticated GitHub CLI) so labels can be audited at any time:

• Checks the PRs merged after a given release (php admin/check-pr-labels.php 4.7.3), defaulting to the latest release. The release's publish timestamp is resolved automatically, so there are no dates to remember.
• Suggests the expected label from the PR title type: fix: → bug, feat: → new feature, perf: → enhancement, refactor: → refactor. Titles typed chore:/ci:/docs:/style:/test: need no changelog label and are ignored.
• Applies the project's labeling nuances: the refactor suggestion is made only when the PR actually changes files under system/ (test-only refactors may use the testing label instead), and release process PRs ("Prep for x.y.z release", "x.y.z Ready code", "x.y.z Merge code") are skipped since they carry no labels.
• PRs with no changelog label and no recognized title type are listed separately for manual review.
• Prints a ready-to-run gh pr edit ... --add-label command per finding, so after reviewing the list the labels can be applied by pasting the block (minus any unwarranted lines).
• On a TTY, output is colored and PR numbers are clickable OSC 8 hyperlinks. Piped output is plain text, and the command block is always unstyled for clean copying.
• Exits 1 when suggested-label findings exist, so it can gate automation later.

Also included:

• admin/RELEASE.md mentions running the script at any time in the Changelog section.
• admin/README.md documents the script.
• tests/system/AutoReview/CheckPrLabelsTest.php covers the usage errors offline and the default/versioned happy paths via gh, with the same explicit CI-skip message pattern as GenerateChangelogTest.

The script has already paid for itself: a run since v4.7.3 surfaced 20+ unlabeled feat: PRs targeting 4.8, two fix: PRs missing bug, and one system/-touching refactor: PR. The suggested labels have since been applied using the script's emitted commands, so a fresh run against the same release now reports no findings.

Checklist:

• Securely signed commits
• Component(s) with PHPDoc blocks, only if necessary or adds value (without duplication)
• Unit testing, with >80% coverage
• User guide updated
• Conforms to style guide