Add firstrun audit: find stale docs and features with no docs

firstrun audit owner/repo reads the git history from one blobless clone:

• ranks every docs page by the date of its last edit
• marks a page stale when product commits on its topic landed after that edit and Jev expects one to break it, and quiet otherwise
• checks each user-facing commit from 120 days against the closest docs page, with Jev judging coverage
• returns proposal cards (document, refresh, feature, content, automate) and drafts release notes and product highlights

--docs-repo reads the docs from a second repo. firstrun audit-issue writes a tracking issue body, and examples/github-action runs the audit weekly. The site gets audit.html, a landing section, a nav link, and published audits of dbt, Altimate Code, Entire CLI and Typer.