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

gitea/post_review_comments_test.go

@@ -37,7 +37,7 @@ func TestPostReview_WithComments(t *testing.T) {
 		{Path: "util.go", NewPosition: 10, Body: "[MINOR] Style issue"},
 	}
 
-	_, err := client.PostReview(context.Background(), "owner", "repo", 1, "REQUEST_CHANGES", "summary", comments)
+	_, err := client.PostReview(context.Background(), "owner", "repo", 1, "REQUEST_CHANGES", "summary", "abc123", comments)
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}
@@ -72,7 +72,7 @@ func TestPostReview_NilComments(t *testing.T) {
 	defer server.Close()
 
 	client := NewClient(server.URL, "test-token")
-	_, err := client.PostReview(context.Background(), "owner", "repo", 1, "APPROVED", "all good", nil)
+	_, err := client.PostReview(context.Background(), "owner", "repo", 1, "APPROVED", "all good", "", nil)
 	if err != nil {
 		t.Fatalf("unexpected error: %v", err)
 	}