chore: import upstream snapshot with attribution

vitest.config.ts

@@ -0,0 +1,10 @@
+import { defineConfig } from 'vitest/config'
+
+export default defineConfig({
+  test: {
+    // Runs once per worker before any test. Scrubs the developer's shell so
+    // session-discovery env vars (CLAUDE_CONFIG_DIRS, HOME, XDG_*, every
+    // provider-specific *_HOME) don't bleed real local data into fixtures.
+    setupFiles: ['./tests/setup/env-isolation.ts'],
+  },
+})