New audit: old actions

[woodruffw]

Old actions are a code smell and potential weakness, since they suggest parts of the CI/CD system that have been forgotten about/are not well-exercised.

It would be nice to have an old-actions or similar audit that flags any uses: clause that contains a branch or tag reference that's N months (6? 12?) older than the newest branch/tag on the associated repo.

This should be an "auditor" persona-only audit, since it'll be noisy and primarily of use for human review.

[woodruffw]

(There are some open design questions here, so if someone is interested in tackling this please respond here first so we can work them out!)