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/plugin.json

@@ -3,13 +3,35 @@
   "name": "示例插件",
   "version": "0.1.0",
   "apiVersion": "1.0",
-  "requires": { "network": ["api.example.com"] },
+  "requires": {
+    "network": [
+      "api.example.com"
+    ]
+  },
   "provides": {
-    "tools": ["http_echo"],
-    "intents": ["demo"],
-    "panels": ["demo"],
-    "cards": ["note"],
-    "prompts": ["assistant"],
-    "settings": ["greeting"]
+    "tools": [
+      "http_echo"
+    ],
+    "intents": [
+      "demo"
+    ],
+    "panels": [
+      "demo"
+    ],
+    "cards": [
+      "note"
+    ],
+    "prompts": [
+      "assistant"
+    ],
+    "settings": [
+      "greeting"
+    ],
+    "workspaces": [
+      "main"
+    ],
+    "commands": [
+      "demo"
+    ]
   }
 }