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>

.gitignore

@@ -36,4 +36,8 @@ tmp/
 *.tmp
 .vscode/
 .claude/settings.local.json
-.paperclip-local/
+.paperclip-local/
+
+# Playwright
+tests/e2e/test-results/
+tests/e2e/playwright-report/