feat(vcs): add CommitID to ReviewRequest (#115)

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

vcs/types.go

@@ -98,6 +98,5 @@ type ReviewRequest struct {
 	// If empty, the platform defaults to the current PR head.
 	// Adapters use this as the primary commit anchor for the review submission.
 	CommitID string `json:"commit_id,omitempty"`
-
 	Comments []ReviewComment `json:"comments,omitempty"`
 }