Prevent nested session

I mentioned this in a comment on #67, but I think nested sessions should be prevented. For example, if I run `desk . project-one` then try to run `desk . project-two`, desk should ask if I want to leave the first session to enter the next or prevent me from nesting at all.

Start at the `desk` command entry point and trace how an existing session is detected when running `desk . project-one` followed by `desk . project-two`. Confirm the intended behavior with the maintainers, then make the second invocation either prompt before leaving the first session or prevent nesting, with coverage for the chosen behavior.

Written by the indexing model from the issue text.