Add Playwright E2E testing infrastructure

- Add playwright-web-flake to flake.nix for NixOS browser support
- Pin @playwright/test@1.56.1 to match nixpkgs version
- Create playwright.config.ts with Chromium-only, auto-start dev server
- Add e2e/smoke.spec.ts with initial smoke tests
- Add .mcp.json for Claude browser control via MCP
- Update .gitignore for playwright artifacts
- Remove E2E test skip from spec.md Known Limitations
- Update specs/testing.md to require three-tier testing approach

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

e2e/smoke.spec.ts

@@ -0,0 +1,36 @@
+// ABOUTME: Smoke tests to verify basic application functionality.
+// ABOUTME: Tests that the app loads and critical pages are accessible.
+import { expect, test } from "@playwright/test";
+
+test.describe("smoke tests", () => {
+  test("app loads with correct title", async ({ page }) => {
+    await page.goto("/");
+
+    // Verify the app loads by checking the page title
+    await expect(page).toHaveTitle("PhaseFlow");
+  });
+
+  test("login page is accessible", async ({ page }) => {
+    await page.goto("/login");
+
+    // Verify login page loads
+    await expect(page).toHaveURL(/\/login/);
+  });
+
+  test("unauthenticated root redirects or shows login option", async ({
+    page,
+  }) => {
+    await page.goto("/");
+
+    // The app should either redirect to login or show a login link
+    // Check for either condition
+    const url = page.url();
+    const hasLoginInUrl = url.includes("/login");
+    const loginLink = page.getByRole("link", { name: /login|sign in/i });
+
+    // At least one should be true: either we're on login page or there's a login link
+    if (!hasLoginInUrl) {
+      await expect(loginLink).toBeVisible();
+    }
+  });
+});