Without credit, fewer sandboxes at once, and a few new limits

Four sandboxes run at once, not eight, with at most 8 vCPU and 16 GiB in all, not 16 and 32. An account keeps at most 10 sandboxes, running, paused or stopped (a create past it is refused with no_credit_kept_limit, 409), and makes at most 50 a day, in any 24 hours (no_credit_daily_limit, 429). Waking, resuming or deleting a sandbox you already keep is never refused. Scheduled jobs do not run without credit: a job waits with blockedReason credits until the account holds credit. Adding credit lifts all of these. Public previews and ports other than 80 and 443 were already for paid accounts only. The Python SDK's Inspect integration defaults to four sandboxes at a time.