feat(core-events): EventDescriptor + defineEvent

TDD: red (test-only), then green. defineEvent(name, schema) returns a typed
EventDescriptor<TName, TSchema>. Also added @repo/core-testing to devDeps
(required for nodeVitestConfig's setupFiles). 3 tests passing.

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

packages/core-events/src/event-descriptor.ts

@@ -0,0 +1,13 @@
+import type { z } from "zod";
+
+export type EventDescriptor<TName extends string, TSchema extends z.ZodType> = {
+  readonly name: TName;
+  readonly schema: TSchema;
+};
+
+export function defineEvent<TName extends string, TSchema extends z.ZodType>(
+  name: TName,
+  schema: TSchema,
+): EventDescriptor<TName, TSchema> {
+  return { name, schema };
+}