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/api.ts

@@ -13,6 +13,7 @@ import { SkillGroup } from "./groups/skill"
 import { EventGroup, makeEventGroup } from "./groups/event"
 import type { Definition } from "@opencode-ai/schema/event"
 import { AgentGroup } from "./groups/agent"
+import { PluginGroup } from "./groups/plugin"
 import { HealthGroup } from "./groups/health"
 import { PtyGroup } from "./groups/pty"
 import { ShellGroup } from "./groups/shell"
@@ -42,6 +43,7 @@ const makeApiFromGroup = <
     .add(HealthGroup)
     .add(LocationGroup.middleware(locationMiddleware))
     .add(AgentGroup.middleware(locationMiddleware))
+    .add(PluginGroup.middleware(locationMiddleware))
     .add(makeSessionGroup(sessionLocationMiddleware))
     .add(MessageGroup.middleware(sessionLocationMiddleware))
     .add(ModelGroup.middleware(locationMiddleware))

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",

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.",
+    }),
+  )