chore(01-01): wire Vitest (happy-dom) and Playwright config + sentinel test

- vitest.config.ts: happy-dom environment (so Plan 03's IndexedDB tests
  can layer fake-indexeddb on top of happy-dom's window per RESEARCH);
  passWithNoTests:false enforces RESEARCH CI Pitfall B (a green CI run
  must mean tests *ran*, not 'no tests existed'); include glob covers
  src/**/*.test.ts(x) and scripts/**/*.test.mjs.
- playwright.config.ts: testDir 'tests/e2e' (not yet created — first spec
  lands in Phase 2 PIPE-07); webServer config wires npm run dev so smoke
  tests can self-start the dev server; baseURL pinned to vite default.
- src/__sentinel__.test.ts: a single test asserting 1+1===2 AND that
  globalThis.window exists, proving the runner is wired and happy-dom is
  active. To be deleted once real tests exist (Plan 03 onward).
- npm test → 1 file, 1 test passed in 593ms.
- npx playwright --version → Version 1.59.1 (matches RESEARCH lock).

vitest.config.ts

@@ -0,0 +1,18 @@
+import { defineConfig } from 'vitest/config';
+
+// Phase 1 test infrastructure config.
+//
+// `happy-dom` is the chosen DOM env because Plan 03's IndexedDB tests will
+// use happy-dom together with `fake-indexeddb/auto` (happy-dom does not ship
+// IndexedDB). RESEARCH § Validation Architecture explicitly calls for this.
+//
+// `passWithNoTests: false` enforces RESEARCH CI Pitfall B — a green CI run
+// must mean tests *ran*, not "no tests existed".
+export default defineConfig({
+  test: {
+    environment: 'happy-dom',
+    include: ['src/**/*.test.ts', 'src/**/*.test.tsx', 'scripts/**/*.test.mjs'],
+    passWithNoTests: false,
+    globals: false,
+  },
+});