Billing money rules

This skill activates when work is happening in services/billing. It exists so money bugs never ship.

The rules (load these every time)

1. Money is integer cents. Never a float, never dollars. Variables and fields end in _cents.
2. Plan economics live in one place — _SEAT_LIMITS and _PRICE_PER_SEAT_CENTS in subscriptions.py. Never hard-code a price or a limit anywhere else.
3. Every money/plan violation raises BillingError (HTTP 402). Never silently clamp seats, skip a charge, or return a partial result.
4. Inactive subscriptions cannot be invoiced.

When you need the detail