Bare 'party host' now walks staff, users, wheel, party and gates on
the first group the caller belongs to, announcing the pick. Boxes
where people already share a system group need no setup; the curated
'party' group remains the fallback and the explicit --group /
TMUX_PARTY_GROUP paths are unchanged. Caller membership is the only
probe (id -nG, portable everywhere); whether a guest is a member stays
an invite-time warning, where it can actually be answered.
