Agent Skills: payment-integration

Load PROACTIVELY when task involves payments, billing, or subscriptions. Use when user says \"add payments\", \"integrate Stripe\", \"set up subscriptions\", \"add a checkout flow\", or \"handle billing webhooks\". Covers Stripe, LemonSqueezy, and Paddle integration, checkout sessions, subscription lifecycle management, webhook verification and handling, customer portal, metered billing, refunds, and PCI compliance considerations.