Require a marker so the example critic can reject once

Why: The live check needs the first draft to miss a line the critic can name, then the retry can add it.
Evidence: The prompt does not mention the marker. maxRetries stays 1.
Impact: demo.brief drafts are rejected until they contain 核对标记：P4.
Refs: feature/agent-harness-p4
Co-authored-by: Cursor <cursoragent@cursor.com>

plugins/example/workflows/demo-brief.json

@@ -24,7 +24,7 @@
       "kind": "prompt",
       "agent": "research",
       "prompt": "写一份关于「{{slots.topic}}」的简报，篇幅：{{steps.length.output}}。",
-      "critic": {"criteria": "必须提到主题，并且是一段能读完的简报", "maxRetries": 1}
+      "critic": {"criteria": "必须包含单独一行「核对标记：P4」。没有这一行就不通过，nextAction 请要求补上这一行。", "maxRetries": 1}
     },
     {
       "id": "save",