feat(gitea): pass commit_id explicitly in PostReview (#107)

Add commitID parameter to gitea.Client.PostReview so the review is
anchored to the specific commit that was evaluated. The caller
(cmd/review-bot) already computes evaluatedSHA from pr.Head.Sha;
this wires it through to the Gitea API payload.

When commitID is empty, omitempty drops it from the JSON and Gitea
defaults to the current PR head (backward-compatible).

Closes #107

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", "", 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)
 	}