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/groups/plugin.ts

@@ -0,0 +1,27 @@
+import { Location } from "@opencode-ai/schema/location"
+import { Plugin } from "@opencode-ai/schema/plugin"
+import { Schema } from "effect"
+import { HttpApiEndpoint, HttpApiGroup, OpenApi } from "effect/unstable/httpapi"
+import { LocationQuery, locationQueryOpenApi } from "./location"
+
+export const PluginGroup = HttpApiGroup.make("server.plugin")
+  .add(
+    HttpApiEndpoint.get("plugin.list", "/api/plugin", {
+      query: LocationQuery,
+      success: Location.response(Schema.Array(Plugin.Info)),
+    })
+      .annotateMerge(locationQueryOpenApi)
+      .annotateMerge(
+        OpenApi.annotations({
+          identifier: "v2.plugin.list",
+          summary: "List plugins",
+          description: "Retrieve currently loaded plugins.",
+        }),
+      ),
+  )
+  .annotateMerge(
+    OpenApi.annotations({
+      title: "plugins",
+      description: "Experimental plugin routes.",
+    }),
+  )