test: add vitest unit tests across packages

Cover shared zod schemas, extract traversal/refs/downloads, and tts filename/error handling. Document the test setup in the README and docs.

vitest.config.ts

@@ -0,0 +1,8 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    environment: 'node',
+    include: ['packages/*/src/**/*.test.ts', 'apps/*/src/**/*.test.ts'],
+  },
+});