vkc-e2e-playwright-runbook

Run and debug Playwright E2E in VKC (Playwright, E2E_PORT=3100, E2E_HEALTH_PATH, webServer, test-results/*/error-context.md). Use when adding/changing e2e specs or debugging E2E failures. (키워드= E2E, 플레이라이트, 테스트, 포트 3100, 헬스체크, webServer, 플래키)