Work with the OpenMeter billing package. Use this skill whenever touching invoice lifecycle, billing profiles, customer overrides, invoice line items, gathering invoices, standard invoices, the invoice state machine, billing validation issues, billing-subscription sync, the billing worker, invoice calculation, rating/pricing engine, or tax config on billing objects. Also use when writing or debugging billing integration tests (BaseSuite, SubscriptionMixin), billing adapter (Ent queries), billing HTTP handlers, or the subscription→billing sync algorithm. Trigger this skill for any file under `openmeter/billing/`, `openmeter/billing/worker/`, `openmeter/billing/service/`, `openmeter/billing/adapter/`, `openmeter/billing/rating/`, `test/billing/`, or `cmd/billing-worker/`.