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/workflows/scripts/demo.py

@@ -0,0 +1,6 @@
+import agentwf as wf
+
+now = wf.tool("time_now", {})
+answer = wf.ask("要记下来吗？", [{"id": "yes", "label": "记下", "impact": "会写入一份备忘"}])
+wf.artifact("demo-script.md", "时间 " + now + "\n选择 " + answer)
+wf.step("run", "跑脚本", "done")