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/client/src/generated/client.ts

@@ -4,6 +4,8 @@ import type {
   LocationGetOutput,
   AgentListInput,
   AgentListOutput,
+  PluginListInput,
+  PluginListOutput,
   SessionListInput,
   SessionListOutput,
   SessionCreateInput,
@@ -310,6 +312,20 @@ export function make(options: ClientOptions) {
           requestOptions,
         ),
     },
+    plugin: {
+      list: (input?: PluginListInput, requestOptions?: RequestOptions) =>
+        request<PluginListOutput>(
+          {
+            method: "GET",
+            path: `/api/plugin`,
+            query: { location: input?.["location"] },
+            successStatus: 200,
+            declaredStatuses: [401, 400],
+            empty: false,
+          },
+          requestOptions,
+        ),
+    },
     session: {
       list: (input?: SessionListInput, requestOptions?: RequestOptions) =>
         request<SessionListOutput>(

packages/client/src/generated/types.ts

@@ -168,6 +168,21 @@ export type AgentListOutput = {
   }>
 }
 
+export type PluginListInput = {
+  readonly location?: {
+    readonly location?: { readonly directory?: string | undefined; readonly workspace?: string | undefined } | undefined
+  }["location"]
+}
+
+export type PluginListOutput = {
+  readonly location: {
+    readonly directory: string
+    readonly workspaceID?: string
+    readonly project: { readonly id: string; readonly directory: string }
+  }
+  readonly data: ReadonlyArray<{ readonly id: string }>
+}
+
 export type SessionListInput = {
   readonly workspace?: {
     readonly workspace?: string | undefined