fix(runtime): sync plugin status after initialization in register_plugin

The refreshed container's status was not copied back, leaving the plugin
in UNMOUNTED state on the runtime side. This caused list_knowledge_engines
(and similar status-gated queries) to skip fully initialized plugins.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: RockChinQ

src/langbot_plugin/runtime/plugin/mgr.py

@@ -426,6 +426,7 @@ async def register_plugin(
         refreshed = runtime_plugin_container.PluginContainer.from_dict(plugin_container_data)
         plugin_container.components = refreshed.components
         plugin_container.manifest = refreshed.manifest
+        plugin_container.status = refreshed.status
 
     async def remove_plugin_container(
         self,