fix: classify unauthorized tmux connections; stop misreporting membership

tmux answers non-allowlisted users with 'access not allowed' on stderr
and exit 0 for every command (verified 3.3a/3.5a, two users), so
has-session reports any target as existing. party status claimed
uninvited group members were joined, party leave silently 'succeeded',
and party list printed bogus '0 attendee(s)' rows. New party_conn_state
(ok/unauthorized/dead) classifies by message content; list now shows
'invite-only (ask <host>)'.

tests/20-helpers.bats

@@ -136,3 +136,22 @@ setup() {
   [ "$status" -ne 0 ]
   [[ "$output" == *"pass a name"* ]]
 }
+
+@test "party_conn_state classifies ok / unauthorized / dead" {
+  stub="$PARTY_TMP/conn-stub"
+  cat > "$stub" <<'S'
+#!/bin/sh
+case "${MODE:-}" in
+  ok)   exit 0 ;;
+  deny) echo "access not allowed" >&2; exit 0 ;;
+  *)    echo "error connecting to /x (No such file or directory)" >&2; exit 1 ;;
+esac
+S
+  chmod +x "$stub"
+  export PARTY_TMUX="$stub"
+  export MODE=ok;   [ "$(party_conn_state /x)" = "ok" ]
+  # The trap this function exists for: tmux answers unauthorized users
+  # with EXIT 0 + a stderr message, so rc alone says "authorized".
+  export MODE=deny; [ "$(party_conn_state /x)" = "unauthorized" ]
+  export MODE=dead; [ "$(party_conn_state /x)" = "dead" ]
+}