Review model

A review belongs to one target and stores file marks, line and file comments, overall notes, screenshot annotations, and a verdict. File marks use content hashes. If a file changes after review, Fettle shows that mark as stale without deleting it.

An exported review packet carries this context back to an agent. Export doesn't alter the repository. Posting to GitHub is deliberately narrower: Fettle can add one comment review to an existing pull request, but can't approve, request changes, merge, or push.