Write the user unit and enable it when systemd is true.
When systemd is false, runs disable --now on the user unit. Skips rewrite when the unit file already matches. Skips enable --now when already active or when this binary is ollmfilesd (the unit already started us). ExecStart is ollmfilesd on PATH. The unit sets OLLMFILESD_DAEMON=1 so the process does not double-fork under Type=simple.