fix(vcs): thread CommitID through abstraction layer (#114)

Add CommitID field to vcs.ReviewRequest so the commit anchor propagates
through the vcs.Client interface to platform adapters.

Changes:
- vcs/types.go: Add CommitID string field to ReviewRequest
- gitea/client.go: Add commitID parameter to PostReview, include in API payload
- gitea/adapter.go: Pass req.CommitID to underlying client
- github/review.go: Use req.CommitID as primary, fall back to comment-level
- cmd/review-bot/main.go: Set CommitID on ReviewRequest from pr.Head.SHA

Fixes #114

cmd/review-bot/main.go

@@ -485,6 +485,7 @@ func main() {
 	reviewReq := vcs.ReviewRequest{
 		Body:     reviewBody,
 		Event:    event,
+		CommitID: pr.Head.SHA,
 		Comments: inlineComments,
 	}
 	posted, err := client.PostReview(ctx, owner, repoName, prNumber, reviewReq)