feat: add Playwright e2e tests for onboarding wizard flow

Scaffolds end-to-end testing with Playwright for the onboarding wizard.
Runs in skip_llm mode by default (UI-only, no LLM costs). Set
PAPERCLIP_E2E_SKIP_LLM=false for full heartbeat verification.

- tests/e2e/playwright.config.ts: Playwright config with webServer
- tests/e2e/onboarding.spec.ts: 4-step wizard flow test
- .github/workflows/e2e.yml: manual workflow_dispatch CI workflow
- package.json: test:e2e and test:e2e:headed scripts

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

tests/e2e/playwright.config.ts

@@ -0,0 +1,35 @@
+import { defineConfig } from "@playwright/test";
+
+const PORT = Number(process.env.PAPERCLIP_E2E_PORT ?? 3100);
+const BASE_URL = `http://127.0.0.1:${PORT}`;
+
+export default defineConfig({
+  testDir: ".",
+  testMatch: "**/*.spec.ts",
+  timeout: 60_000,
+  retries: 0,
+  use: {
+    baseURL: BASE_URL,
+    headless: true,
+    screenshot: "only-on-failure",
+    trace: "on-first-retry",
+  },
+  projects: [
+    {
+      name: "chromium",
+      use: { browserName: "chromium" },
+    },
+  ],
+  // The webServer directive starts `paperclipai run` before tests.
+  // Expects `pnpm paperclipai` to be runnable from repo root.
+  webServer: {
+    command: `pnpm paperclipai run --yes`,
+    url: `${BASE_URL}/api/health`,
+    reuseExistingServer: !!process.env.CI,
+    timeout: 120_000,
+    stdout: "pipe",
+    stderr: "pipe",
+  },
+  outputDir: "./test-results",
+  reporter: [["list"], ["html", { open: "never", outputFolder: "./playwright-report" }]],
+});