Get My Credits

Return the caller's current credit balance and billing period details. `balance` is the authoritative gate value: use it to decide whether to attempt a workflow run. `remaining` is a display convenience derived from settled ledger entries and may temporarily exceed `balance` while a workflow run holds an open reserve. `used` reflects credits consumed in the current billing period. `plan_grant` is the total monthly credit allowance for the caller's plan, enabling a "X / Y used" display. `period_start` and `period_end` mark the boundaries of the current billing window; free-tier callers use a calendar-month boundary.