Workflow-driven skill that builds payment webhook replay + recovery tests. Covers the idempotency contract (every webhook handler must handle redelivery without side effects), the replay simulators (Stripe CLI `stripe trigger`, Adyen Customer Area resend, PayPal Webhook Simulator, Braintree webhook test), the signature-verification gauntlet (HMAC-SHA256 per gateway, expired-timestamps rejection), and the partial-failure recovery scenarios. Use when designing webhook robustness tests.