Workers start from their coordinator's snapshots

A key made by a key can now create a sandbox from a snapshot that its maker, or any key above it, took and labelled share: made-keys. It can only start from it: it never lists, reads, changes or deletes it, and a snapshot without the label stays out of its reach. The label is never copied to sandboxes, forks or new snapshots made from a shared one: set it on each snapshot you share. Worker keys made before this release need to be made again to use it. A share: made-keys label already on a snapshot was taken off, so nothing became shared by itself; a share label of any other value is untouched. A snapshot of a sandbox that named its egress secrets (secrets) is never shared: workers get snapshot_not_found for it, while its owner still starts it. Sharing never crosses a compartment. Before, a worker key with reach agent was answered snapshot_not_found for every snapshot but its own. See Run a swarm.