chore: add script to sync release branches

[paulbalandan]

Description
Final piece of the release automation series (#10390, #10391, #10392, #10393). RELEASE.md contained three nearly identical fetch/merge/push blocks that the release manager typed out by hand: merging develop into the next minor branch before the release, fast-forwarding develop from master after it, and updating the next minor branch again.

New script admin/sync-release-branches.php:

• Usage: php admin/sync-release-branches.php <target> <source> [--push], e.g. 4.8 develop --push (merges develop into 4.8) or develop master --push (merges master into develop).
• Encodes the sequence once: git fetch upstream, check out <target> (creating it from upstream/<target> if there is no local branch), git merge upstream/<target>, git merge upstream/<source>, and only with the explicit --push flag, git push upstream HEAD. Without --push it stops so the merge result can be reviewed first.
• Guards before touching anything: the branch names must be develop, master, or X.Y, target and source must differ, the upstream remote must be configured, and the working tree must be clean.
• On a merge conflict it exits with instructions to resolve, git merge --continue, and push, leaving the merge in progress for the release manager.

Also included:

• admin/RELEASE.md: the three command blocks collapse to one-line script calls.
• admin/README.md: script documented.
• tests/system/AutoReview/SyncReleaseBranchesTest.php: data-provider coverage of the argument validation. The successful path switches branches and merges, so it is not exercised by the test suite. It was instead verified end-to-end in a disposable sandbox (seed repo, bare "upstream", work clone) covering the --push run, the review-first run, the conflict exit, the dirty-tree guard, and the create-missing-local-branch case.

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