fix: unattended sudo via temporary NOPASSWD drop-in with triple cleanup

Piping a password into ssh -tt's pty raced and hung on the real run (worked in
a fast test, blocked for 14 min in practice). Replace with a temp
/etc/sudoers.d/mac-setup installed by _install_sudoers.sh (validated
before activation), removed on every exit path plus a 40-min on-Mac
self-destruct. setup.sh now fails fast if passwordless sudo is absent
instead of depending on a primed tty ticket.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

scripts/setup.sh

@@ -12,9 +12,7 @@ log(){ printf '\033[1;33m[%s]\033[0m %s\n' "$1" "$2"; }
 # ---------- preflight ----------
 [[ $(uname -m) == arm64 && $(sw_vers -productVersion) == 26.* ]] || { echo "not the Mac this was written for"; exit 1; }
 [[ ${HOSTNAME_WANT:-} ]] || { echo "HOSTNAME_WANT=<name> required"; exit 1; }
-sudo -v   # ticket already primed by run.sh over the pty; prompts if run by hand
-( while true; do sudo -n true; sleep 50; done ) &
-KEEPALIVE=$!; trap 'kill $KEEPALIVE 2>/dev/null' EXIT
+sudo -n true 2>/dev/null || { echo "passwordless sudo unavailable — run via scripts/run.sh (it installs a temp drop-in)"; exit 1; }
 
 # ---------- helpers ----------
 backup_domain(){ # once per domain per run