Persistence can be turned off, and a stopped persistent sandbox deleted

runtime sandbox update <id> --persistent off, update({ persistent: false }) or :update with {"persistent": false} now works as the guides said: a running sandbox stops paying for its disk at once, and a stopped one has its disk deleted and its name freed. A stopped persistent sandbox is listed by runtime ls, runtime sandbox ls and GET /v1/sandboxes with the live ones, since its disk is still kept and billed.