Root cause of recurring breaking bumps: dependabot-auto-merge ran
gh pr merge --auto with no CI gate and the repo has no branch protection,
so PRs merged the moment checks finished — including FAILED ones
(eslint@10, tsparticles@4, typescript@7 all landed repeatedly).
Add a github-script step: list check-runs for the PR head, fail unless
all checks completed AND none failed. Breaking bumps now block instead
of silently landing.