fix: add clear label to PR title check output

- Print '--- PR title check ---' in the log so users can distinguish
  PR title validation from individual commit message checks
- Always set output_prefix for result.txt so the failure output clearly
  identifies which check failed

Author: shenxianpeng

main.py

@@ -246,10 +246,11 @@ def run_commit_check() -> int:
             pr_title = get_pr_title()
             if pr_title:
                 cmd_args = ["--message"]
-                output_prefix = None
+                output_prefix = "# PR Title Validation\n"
                 if emitted_failure_output:
                     cmd_args.append("--no-banner")
-                    output_prefix = "\n---\n# PR Title\n"
+                    output_prefix = "\n---\n# PR Title Validation\n"
+                print("--- PR title check ---")
                 rc = run_check_command(
                     cmd_args,
                     result_file,