feat: auto-select the gating group; 'party' becomes the fallback

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.

ROADMAP.md

@@ -26,11 +26,6 @@ Direction, not promises. Two rules govern everything below (README
 
 ## Later
 
-- **Group ergonomics:** the default `party` group needs root and a
-  relog before anyone's first party. On boxes where everyone already
-  shares a group, `host` could offer (or default to) the caller's
-  primary group when `party` doesn't exist. Fits the trust model;
-  costs a little existence-confidentiality.
 - **Matrix:** live validation on NetBSD and DragonFly; one
   interactive attach/role/switch pass per release (automated coverage
   is all non-interactive).