Fork a briq: a new briq that starts from a copy of its disk

When the briq goes to sleep: after this long without use (agent calls, visits to its stack's URLs). A string: "30m", "3h", "7d" or "never", never a number. A sleeping briq keeps its disk, costs nothing for compute, and the next use wakes it. "never" keeps it always on (Pro plan and above). Left out, the source's. A window longer than this key allows is refused