fix(ci): add TCP keepalive to stunnel to prevent NAT connection resets

Connection drops consistently at ~50s suggest NAT/firewall idle timeout.
Keepalive probes every 10s on the remote side prevent the RST.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

scripts/setup_dagger_remote.sh

@@ -34,6 +34,11 @@ cat << EOF > "$STUNNEL_CONF"
 client = yes
 foreground = yes
 pid = /tmp/stunnel.pid
+; TCP keepalive on the remote side to prevent NAT/firewall from resetting the connection
+socket = r:SO_KEEPALIVE=1
+socket = r:TCP_KEEPIDLE=10
+socket = r:TCP_KEEPINTVL=5
+socket = r:TCP_KEEPCNT=3
 
 [dagger]
 accept = 127.0.0.1:1774