feat(core): add plugin list endpoint

Adds a plugin.list HTTP endpoint that reports currently loaded plugins, wires it through protocol, server, and generated clients, and merges it into sdk-next's plugin API. Also updates the sample plugin to register a primary agent instead of a subagent for local testing.

packages/protocol/src/client.ts

@@ -20,6 +20,7 @@ export const groupNames = {
   "server.health": "health",
   "server.location": "location",
   "server.agent": "agent",
+  "server.plugin": "plugin",
   "server.session": "session",
   "server.message": "message",
   "server.model": "model",