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/package.json

@@ -24,6 +24,7 @@
   },
   "devDependencies": {
     "@repo/core-eslint": "workspace:*",
+    "@repo/core-testing": "workspace:*",
     "@repo/core-typescript": "workspace:*",
     "typescript": "^5.8.0",
     "vitest": "^3.0.0"