Installing on a host should be a command a person pastes, not a user
session copied onto a machine. A token is issued for a team, spent by its
first use, expires after an hour and is stored only as a digest, because
it travels in that command and so lands in shell history.
Redeeming is one conditional update inside the registration transaction:
concurrent redemptions of one token register exactly one machine, and a
registration that fails leaves the token unspent.
