Employees + USB default-disabled with an enable-time provisioning note

Both plugins provision extra tables, so they now install disabled and explain
themselves before a site opts in.

- Plugin contract gains get_provisioning_note() -> {tables, note, docs}.
  Employees and USB implement it (what tables get created in shopdb, how they
  are referenced, link to the schema README; USB references the captured
  DLP/reminder plans).
- Manifest default_enabled=false for employees + usb; the plugins list API
  returns provisioning_note + default_enabled; install now registers a plugin
  disabled when default_enabled is false.
- Setup wizard Features step renders the provisioning note the moment a plugin
  with one is enabled.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

plugins/employees/manifest.json

@@ -7,6 +7,9 @@
   "core_version": ">=0.1.0,<1.0.0",
   "api_prefix": "/api/employees",
   "provides": {
-    "features": ["employee_directory"]
-  }
-}
+    "features": [
+      "employee_directory"
+    ]
+  },
+  "default_enabled": false
+}