Pinned advisory review: fall back to a body-only review on the same commit, not an issue comment #569

Need

When a review pinned to the labeled head (#462, #566) is refused, posting.post_round_review re-reads the head and, if it has not moved, posts the findings as an issue comment. Issue comments carry no commit pin, so a push between that read and the post puts findings for the old head on the new one. The window is one API round trip; the comment's round stamp still names the reviewed head.

Build

Under a pin, the fallback is a review with commit_id set to the same head and the findings in its body (no inline comments), so GitHub itself anchors it to the reviewed commit. Keep the issue-comment fallback for unpinned reviews.