Pin all remaining GitHub Actions references to full-length commit SHAs,
matching the convention already established by third-party actions in
this repo. This is a prerequisite for enabling GitHub's "Require actions
to be pinned to a full-length commit SHA" repository ruleset, which
mitigates tag-hijacking supply chain attacks.
GitHub issue forms enforce `required: true` checkboxes in the web UI,
but the API bypasses form validation — bots and scripts can open issues
with every box unchecked or skip the template entirely. This adds a
workflow that auto-closes those issues, with an org-membership carve-out
so maintainers can still open free-form issues.
