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

github/review.go

@@ -82,21 +82,25 @@ func translateGitHubReviewState(state string) string {
 // (via the omitempty tag on postReviewRequest.Comments).
 //
 // The GitHub API accepts a single commit_id per review submission. PostReview
-// extracts it from the first comment with a non-empty CommitID. If any subsequent
-// comment specifies a different CommitID, PostReview returns ErrConflictingCommitIDs.
-// Comments with an empty CommitID are allowed and inherit the review-level value.
+// uses req.CommitID as the primary commit anchor. If req.CommitID is empty,
+// it falls back to extracting from the first comment with a non-empty CommitID.
+// If any subsequent comment specifies a different CommitID, PostReview returns
+// ErrConflictingCommitIDs. Comments with an empty CommitID are allowed and
+// inherit the review-level value.
 func (c *Client) PostReview(ctx context.Context, owner, repo string, number int, req vcs.ReviewRequest) (*vcs.Review, error) {
 	reqURL := fmt.Sprintf("%s/repos/%s/%s/pulls/%d/reviews",
 		c.baseURL, url.PathEscape(owner), url.PathEscape(repo), number)
 
 	payload := postReviewRequest{
-		Body:  req.Body,
-		Event: string(req.Event),
+		Body:     req.Body,
+		Event:    string(req.Event),
+		CommitID: req.CommitID,
 	}
 
 	// Build the payload in one pass. The GitHub API accepts a single commit_id
-	// per review; we extract it from the first comment that supplies one and
-	// reject the request if any other comment disagrees.
+	// per review. req.CommitID is the primary source; if empty, we extract from
+	// the first comment that supplies one. Reject if any comment disagrees with
+	// the resolved commit_id.
 	for _, comment := range req.Comments {
 		if comment.CommitID != "" {
 			if payload.CommitID == "" {