* ci: pin actions to commit SHAs and add shellcheck
Pin actions/github-script in catalog-assign.yml to a full commit SHA; all
other workflows were already pinned. Add a repo-wide regression test that
every workflow `uses:` ref is pinned to a 40-char commit SHA.
Add a shellcheck job to lint.yml (--severity=error over scripts/bash/*.sh)
and document the local command in CONTRIBUTING.md.
* ci: use repo-standard actions/checkout v7.0.0 in shellcheck job
* ci: shellcheck all tracked shell scripts
Assisted-by: Codex (model: GPT-5, autonomous)
* ci: address workflow hygiene review feedback
Assisted-by: Codex (model: GPT-5, autonomous)
