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>

pnpm-lock.yaml

@@ -531,6 +531,9 @@ importers:
       '@repo/core-eslint':
         specifier: workspace:*
         version: link:../core-eslint
+      '@repo/core-testing':
+        specifier: workspace:*
+        version: link:../core-testing
       '@repo/core-typescript':
         specifier: workspace:*
         version: link:../core-typescript