The remediation-message test relied on the runner not being in group
'party'; on boxes where the runner is (the deployment hosts), host
succeeded and the test failed. Route the child's id(1) through the
suite's stub so the caller is deterministically in no candidate group.
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.
