Feature: add new motd "prompt" display mode

[ethanpailes]

We should add a new "prompt" motd display mode that leverages our prompt hook infrastructure by adding a mechanism to check for and dump out any $XDG_RUNTIME_DIR/shpool/motd.txt file found before deleting it. Then shpool attach can populate this file based on a cooldown the same way we have for pager mode (though the cooldown will be required not optional since we wouldn't want to display this every prompt).