Agent Skills: billing-review

Review a diff that touches Polar's billing domain — subscriptions, cycles and crons, orders, billing entries, meters and usage, discounts, checkout, payments and dunning, refunds, disputes, payouts, wallets, tax and invoices. Use before opening a PR that changes money movement or subscription lifecycle, when the user asks for a billing review, or when a reviewer needs the domain rules the team enforces in review but that no linter catches.