When a release PR merges, the validate step would fail on version drift
that the release PR itself introduced, blocking release-please from
creating tags and GitHub Releases. Detect release PR merges by commit
message prefix and skip validate + stale-close steps so release-please
runs unimpeded.
release-please skips updating the PR body when it finds an existing PR,
causing the changelog to miss commits that landed after the PR was
created. Fix by closing the stale PR before release-please runs so it
always creates a fresh PR with the full changelog.
Also set cancel-in-progress: true so rapid successive merges don't
race to create the PR with partial commit history.
