fix: wire plugin event subscriptions from worker to host

Plugin workers register event handlers via `ctx.events.on()` in the SDK,
but these subscriptions were never forwarded to the host process. The host
sends events via `notifyWorker("onEvent", ...)` which produces a JSON-RPC
notification (no `id`), but the worker only dispatched `onEvent` as a
request handler — notifications were silently dropped.

Changes:
- Add `events.subscribe` RPC method so workers can register subscriptions
  on the host-side event bus during setup
- Handle `onEvent` notifications in the worker notification dispatcher
  (previously only `agents.sessions.event` was handled)
- Add `events.subscribe` to HostServices interface, capability map, and
  host client handler
- Add `subscribe` handler in host services that registers on the scoped
  plugin event bus and forwards matched events to the worker

packages/plugins/sdk/src/host-client-factory.ts

@@ -103,9 +103,10 @@ export interface HostServices {
     list(params: WorkerToHostMethods["entities.list"][0]): Promise<WorkerToHostMethods["entities.list"][1]>;
   };
 
-  /** Provides `events.emit`. */
+  /** Provides `events.emit` and `events.subscribe`. */
   events: {
     emit(params: WorkerToHostMethods["events.emit"][0]): Promise<void>;
+    subscribe(params: WorkerToHostMethods["events.subscribe"][0]): Promise<void>;
   };
 
   /** Provides `http.fetch`. */
@@ -261,6 +262,7 @@ const METHOD_CAPABILITY_MAP: Record<WorkerToHostMethodName, PluginCapability | n
 
   // Events
   "events.emit": "events.emit",
+  "events.subscribe": "events.subscribe",
 
   // HTTP
   "http.fetch": "http.outbound",
@@ -407,6 +409,9 @@ export function createHostClientHandlers(
     "events.emit": gated("events.emit", async (params) => {
       return services.events.emit(params);
     }),
+    "events.subscribe": gated("events.subscribe", async (params) => {
+      return services.events.subscribe(params);
+    }),
 
     // HTTP
     "http.fetch": gated("http.fetch", async (params) => {