Add CommitID string field to vcs.ReviewRequest so both platform
adapters can thread the commit anchor through the abstraction layer.
Changes:
- vcs/types.go: Add CommitID field with json tag commit_id,omitempty
- gitea/client.go: Re-add commitID parameter to PostReview (was
removed during PR #112 refactoring)
- gitea/adapter.go: Forward req.CommitID to underlying client
- github/review.go: Use req.CommitID as primary anchor, fall back to
comment-derived CommitID when empty, reject on conflict
- cmd/review-bot/main.go: Set ReviewRequest.CommitID = evaluatedSHA
Fixes#115