Free machine time in GET /v1/usage matches what a create is admitted on

trial.availableMs now counts what running sandboxes hold and the free-usage rules, so a create without credit is refused with included_usage_used_up exactly when it is 0. The new freeUsageHold field says why an account has no free machine time (hosting, region, card, withheld, no-source or unknown) and what starts it; the refusal's details.reason names the same rule, and npx withruntime usage shows the message. Before, an account could read free time left while every create was refused.