fix: use getContentMerklePayload to extract prompt text

Was showing raw YAML of the CAS node instead of the payload string.

小橘 <xiaoju@shazhou.work>

packages/cli-workflow/src/commands/serve/routes-thread.ts

@@ -27,10 +27,7 @@ async function readStartInfo(
   const promptHash = parsed.node.refs[0] ?? null;
   let prompt: string | null = null;
   if (promptHash !== null) {
-    const promptYaml = await cas.get(promptHash);
-    if (promptYaml !== null) {
-      prompt = promptYaml;
-    }
+    prompt = await getContentMerklePayload(cas, promptHash);
   }
   return { name, prompt };
 }