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/server/src/handlers.ts

@@ -10,6 +10,7 @@ import { CommandHandler } from "./handlers/command"
 import { SkillHandler } from "./handlers/skill"
 import { EventHandler } from "./handlers/event"
 import { AgentHandler } from "./handlers/agent"
+import { PluginHandler } from "./handlers/plugin"
 import { HealthHandler } from "./handlers/health"
 import { PtyHandler } from "./handlers/pty"
 import { ShellHandler } from "./handlers/shell"
@@ -26,6 +27,7 @@ export const handlers = Layer.mergeAll(
   HealthHandler,
   LocationHandler,
   AgentHandler,
+  PluginHandler,
   SessionHandler,
   MessageHandler,
   ModelHandler,

packages/server/src/handlers/plugin.ts

@@ -0,0 +1,13 @@
+import { PluginV2 } from "@opencode-ai/core/plugin"
+import { Effect } from "effect"
+import { HttpApiBuilder } from "effect/unstable/httpapi"
+import { Api } from "../api"
+import { response } from "../location"
+
+export const PluginHandler = HttpApiBuilder.group(Api, "server.plugin", (handlers) =>
+  handlers.handle("plugin.list", () =>
+    Effect.gen(function* () {
+      return yield* response(PluginV2.Service.use((plugin) => plugin.list()))
+    }),
+  ),
+)