Guard: Enable tmux notification only if available
Guard: Enable tmux notification only if available

Check for envvar TMUX otherwise guard will break with:

    :tmux notifier is only available inside a TMux session.
    (Notiffany::Notifier::Base::UnavailableError)

[ci skip]

solnic/transproc,mhib/transproc,polypressure/transproc,pap/transproc,adz/transproc