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/schema/src/plugin.ts

@@ -6,6 +6,11 @@ import { define, inventory } from "./event"
 export const ID = Schema.String.pipe(Schema.brand("Plugin.ID"))
 export type ID = typeof ID.Type
 
+export interface Info extends Schema.Schema.Type<typeof Info> {}
+export const Info = Schema.Struct({
+  id: ID,
+}).annotate({ identifier: "Plugin.Info" })
+
 const Added = define({
   type: "plugin.added",
   schema: { id: ID },