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

vcs/types.go

@@ -93,6 +93,10 @@ type ReviewRequest struct {
 	// Body is the top-level review comment.
 	Body string `json:"body"`
 	// Event is the review action (approve, request changes, or comment).
-	Event    ReviewEvent     `json:"event"`
+	Event ReviewEvent `json:"event"`
+	// CommitID anchors the review to a specific commit SHA.
+	// 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"`
 }