shell_sage does not gracefully upgrade pre-existing users, when it upgrades its preferred model.

I encountered this issue recently: https://github.com/AnswerDotAI/shell_sage/issues/59

## problem

When a user runs shell sage, it saves a conf file which describes the user's preferred model as shell_sage's current preferred default model.

As time passes, the current preferred model changes, but the user's preferred model remains locked to the one save in their conf file.

Eventually, the model saved in the user's conf fill will be retired, and then shell sage will break for that user.

This happened to me with sonnet 3.5.

## possible solution

update the conf format so that it can encode that the user's preferred model is "whatever shell_sage uses as the default right now" rather than some specific value which was the shell_sage default in the past.