CI: add all-checks-passed gate job

[jnasbyupgrade]

Have claude setup branch protection when this merges!

Summary

• Adds a stable all-checks-passed job (needs: [test], if: always()) as a single required-status-check name that survives future job renames/matrix changes, instead of requiring a real job name directly. Mirrors Postgres-Extensions/pg_count_nulls' current ci.yml pattern, including its self-check step that fails loudly if a new job is ever added to jobs: without also being added to all-checks-passed's own needs: list.
• Drops paths-ignore: '**.md' from both triggers. With it, a doc-only push/PR never runs this workflow at all -- so a required all-checks-passed check would never report on one and would sit stuck Pending in branch protection forever, instead of passing. test is one cheap Perl job with no matrix to skip for cost reasons, so there's no real tradeoff in just always running it -- same reasoning pg_count_nulls' own changes job documents for why it must run unconditionally.
• This PR only adds the job -- it does not itself configure branch protection, since that's a live repo setting (gh api --method PUT .../branches/main/protection), not something a git merge can apply. Postgres-Extensions/pg_count_nulls had no branch protection on master at all, so a PR could show mergeStateStatus: CLEAN even with zero completed check-runs; the same gap currently exists here.

Test plan

• This change only affects a GitHub Actions workflow file.
• After merge: configure branch protection on main requiring the all-checks-passed context, then confirm on an open PR that mergeStateStatus reflects a real pending/completed run rather than showing CLEAN with no check having run.

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