Create a board execution (general-purpose container for sections).

UNMETERED, and CreditSku.NONE is typed out rather than defaulted so that stays a decision. This is the twin-chat node persisting a chat that has ALREADY finished — the client posts status: "completed" — so every model call it could pay for was made and paid for somewhere else. The ledger still writes the delta=0 row: that is the execution's idempotency record, and it is what makes a later refund a well-defined zero movement instead of an unexplained 'no_deduct'.