feat(core-events): public barrel

Export EventDescriptor, defineEvent, IEventBus, EventHandler,
CORE_EVENTS_SYMBOLS, InMemoryEventBus, InMemoryEventBusOptions, and
PayloadJobsEventBus from src/index.ts. All typecheck, test (11 tests),
and lint checks pass. turbo boundaries: 454 files, no issues.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/core-events/src/index.ts

@@ -0,0 +1,6 @@
+export type { EventDescriptor } from "./event-descriptor";
+export { defineEvent } from "./event-descriptor";
+export type { IEventBus, EventHandler } from "./event-bus.interface";
+export { CORE_EVENTS_SYMBOLS } from "./symbols";
+export { InMemoryEventBus, type InMemoryEventBusOptions } from "./in-memory-event-bus";
+export { PayloadJobsEventBus } from "./payload-jobs-event-bus";