models no longer hard coded

nb - breaking change to the command line interface.

It doesn't make sense to track all of the models each of the providers releases. It's too fast a moving target. In particular with Ollama where the user can supply their own models which we have no way of knowing about, requiring them to edit the source code of this program if they want to use them.

The README and the sandbox makefile are both updated to match the new interface.