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/sdk-next/src/opencode.ts

@@ -44,7 +44,7 @@ export const create = Effect.fn("OpenCode.create")(function* () {
     // `PluginContext`, so `ctx.agent.transform` and every other hook behave exactly
     // as they do for a config-discovered plugin. Define agent profiles here at
     // startup, then select one per Session with `sessions.create({ agent })`.
-    plugin: plugins.register,
+    plugin: Object.assign(plugins.register, client.plugin),
   }
 })