Adds an `actionlint` job that runs on changes to `.github/workflows/**`
and `.github/actions/**`. Catches workflow bugs at PR time — expression
typos, deprecated runner labels, broken matrices, and shellcheck issues
in `run:` blocks.
Run from the official `docker://rhysd/actionlint` image, digest-pinned
alongside everything else.
Existing workflows had 6 shellcheck findings, all fixed.
nicktrn
b19cf6df25