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

server/src/services/plugin-host-services.ts

@@ -556,6 +556,17 @@ export function buildHostServices(
         }
         await scopedBus.emit(params.name, params.companyId, params.payload);
       },
+      async subscribe(params: { eventPattern: string; filter?: Record<string, unknown> }) {
+        scopedBus.subscribe(
+          params.eventPattern as any,
+          params.filter as any ?? {},
+          async (event) => {
+            if (notifyWorker) {
+              notifyWorker("onEvent", { event });
+            }
+          },
+        );
+      },
     },
 
     http: {