deploy: gate cockpit block on ENABLE_COCKPIT

SKIP_COCKPIT only existed in the deleted deploy.conf; under set -u the
cockpit block would abort with an unbound variable.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

deploy.sh

@@ -283,7 +283,7 @@ install -D -m0644 modem/van-sms-watch.service /etc/systemd/system/van-sms-watch.
 # are PolicyKit-gated). Not a service, just a tool: sudo van-sms-send <number> <text>
 install -D -m0755 modem/van-sms-send /usr/local/sbin/van-sms-send
 
-if [ "$SKIP_COCKPIT" -ne "1" ]; then
+if [ "$ENABLE_COCKPIT" = 1 ]; then
     echo "== cockpit plugin =="
     install -d /usr/share/cockpit/vanrouter
     for f in cockpit/vanrouter/*; do