Requires openssh; creates ~/.ssh (0700, owned by the operator account via
${machine:account}); writes every other node's Host block (HostName + User
<account>) into a marked region of ~/.ssh/config (home-scoped, into:block), so
`ssh <node>` reaches each peer as the right account and the operator's own
config is kept. Universal-tier: assigned wherever a person logs in; a node with
no account gets no config.