Pin the T4 workspaces combo and extend the example plugin (overlay shell, pure inbox rail)

Why: T4 submodule PRs merged (backend #11, ui #8, frontend #6->#7, documents #12) and the Dev combo must pin the verified combination; the example plugin gains workspace and command declarations exercising the new surfaces.
Evidence: combo make test PASS at pinned heads; documents docs/records/2026-09-30-t4-workspaces-verification.md (PR#12); example assets JSON-validated.
Impact: gitlinks backend=303cdf3 frontend=e5b65ca ui=1c2ddec documents=<PR#12 head>; plugins/example adds workspaces/main.json + commands/demo.json + manifest provides; task card backfilled.
Refs: task card feature/workspaces-r1; proposal T4

plugins/example/workspaces/main.json

@@ -0,0 +1,12 @@
+[
+  {
+    "id": "example.governance",
+    "title": "治理总览（示例）",
+    "icon": "🛡",
+    "shortcut": "9",
+    "body": [
+      { "type": "markdown", "markdown": "**示例插件工作区**——声明式 Body（UI 有限词汇），经 OverlayHost 渲染。" },
+      { "type": "kv", "kv": [["来源", "plugins/example/workspaces/main.json"], ["快捷键", "⌘9"], ["关闭", "Esc / 遮罩点击"]] }
+    ]
+  }
+]