fix: postPrReview falls back to a plain comment when self-review is blocked

[lsfera]

Summary

While manually re-triggering a review on PR #96 (to validate #98's reviewer-auth fix), posting the review result to GitHub failed. Root cause, confirmed via a raw gh api call:

{"message":"Unprocessable Entity","errors":["Review Can not request changes on your own pull request"],...}

GitHub's REST API rejects REQUEST_CHANGES reviews from a PR's own author — the normal case for this repo, since the orchestrator's GH_TOKEN both opens every PR and posts its AI review. Both postPrReview's inline-comment attempt and its existing body-only fallback hit the identical 422 (the restriction is about the review event type, not inline positioning), so every changes-requested verdict has silently failed to post, historically. This doesn't compromise safety — auto-merge is gated by the reducer's own in-memory state, not GitHub's review status — but it does mean the AI's feedback has never actually been visible to a human until I posted one manually as a workaround.

Change

postPrReview now tries, in order:

1. inline review (existing)
2. body-only review, no inline comments (existing)
3. new: a plain PR comment with the same verdict + summary + per-file comments (formatReviewAsComment, a new pure helper) — plain comments have no self-review restriction

Test plan

• npm test — 209/209 passing (3 new tests for formatReviewAsComment)
• tsc --noEmit — clean
• Live-verified against PR #84: Headroom × sandcastle: Phase 1 validation — measure token savings on a live run #96: both review-posting attempts 422'd exactly as predicted, and the plain-comment fallback posted successfully