chore(core-events): scaffold package

Create @repo/core-events package skeleton with package.json, tsconfig.json,
vitest.config.ts (using mergeConfig with nodeVitestConfig), eslint.config.js,
turbo.json (tagged core), and AGENTS.md. Register workspace via pnpm install.

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

packages/core-events/turbo.json

@@ -0,0 +1,4 @@
+{
+  "extends": ["//"],
+  "tags": ["core"]
+}