'shell' is a child of the exec connection: drop the terminal and a long
Claude run inside it is SIGHUP'd — which has already cost one run. tmux
is installed in every box but relying on remembering to start it is how
that run was lost.
New verb, not a changed default: 'tmux new-session -A -s <session>'
attaches when the session exists and creates it when it doesn't, so
starting work and reattaching after a disconnect are the same command.
The session name (default: main) gives parallel streams in one box.
'shell' stays bare on purpose — two verbs, two contracts — so tmux's
costs (nesting, scrollback semantics, exit-vs-detach) are only paid
when asked for.
Closes#6
