Add example brief and script workflows

Why: The example plugin needs a bound intent and a plugin-shipped script so the three P4 paths can be checked against a real plugin.
Evidence: TestRepoExamplePluginLoads accepts the plugin, asks 主题是什么？, and keeps the topic slot.
Impact: demo.brief starts the five-step workflow. demo.script stays unbound and needs runtime.workflowScripts.
Refs: feature/agent-harness-p4
Co-authored-by: Cursor <cursoragent@cursor.com>

plugins/example/plugin.json

@@ -32,6 +32,10 @@
     ],
     "commands": [
       "demo"
+    ],
+    "workflows": [
+      "demo-brief",
+      "demo-script"
     ]
   }
 }