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/worker-rpc-host.ts

@@ -387,6 +387,13 @@ export function startWorkerRpcHost(options: WorkerRpcHostOptions): WorkerRpcHost
             registration = { name, filter: filterOrFn, fn: maybeFn };
           }
           eventHandlers.push(registration);
+          // Register subscription on the host so events are forwarded to this worker
+          void callHost("events.subscribe", { eventPattern: name, filter: registration.filter ?? null }).catch((err) => {
+            notifyHost("log", {
+              level: "warn",
+              message: `Failed to subscribe to event "${name}" on host: ${err instanceof Error ? err.message : String(err)}`,
+            });
+          });
           return () => {
             const idx = eventHandlers.indexOf(registration);
             if (idx !== -1) eventHandlers.splice(idx, 1);
@@ -1107,6 +1114,14 @@ export function startWorkerRpcHost(options: WorkerRpcHostOptions): WorkerRpcHost
         const event = notif.params as AgentSessionEvent;
         const cb = sessionEventCallbacks.get(event.sessionId);
         if (cb) cb(event);
+      } else if (notif.method === "onEvent" && notif.params) {
+        // Plugin event bus notifications — dispatch to registered event handlers
+        handleOnEvent(notif.params as OnEventParams).catch((err) => {
+          notifyHost("log", {
+            level: "error",
+            message: `Failed to handle event notification: ${err instanceof Error ? err.message : String(err)}`,
+          });
+        });
       }
     }
   }