The setup-complete marker is a runtime-state file, not config, so move it
to /opt/so/state/setup-complete. Updates both writers (mark_setup_complete
in setup/so-functions and the upgrade-path state in minion/init.sls) and the
three readers (so-boot-highstate.service ConditionPathExists, boot_highstate.sls
enable gate, and the so-user_sync cron gate).