feat(01-01): configure Vitest for pure-function unit tests

- Created vitest.config.ts with environment: 'node' and globals: true
- Added passWithNoTests: true so vitest run exits 0 with no test files
- test script already present in package.json as "vitest run"

vitest.config.ts

@@ -0,0 +1,9 @@
+import { defineConfig } from 'vitest/config';
+
+export default defineConfig({
+  test: {
+    environment: 'node',
+    globals: true,
+    passWithNoTests: true,
+  },
+});