because we prepare sh env in tmuxinator, they're invisible after detach/attach.
update-environment will set PROJECT env to null if it can't see it when attaching.

New schema is set PROJECT to tmux session in tmuxinator.
* tmux store it in global session. new window/pane will inherit, and tmux will set sh env based on session env.
* After detach/attach, global session env can be restored.