This PR will:

Move models environment into package namespace and don't load them during package load. This is simpler than what we are doing now.

1. In R/models.R:
   models <- new.env()

2. In load_models():

   • Drop code that creates the environment.
   • Update conditional for loading models to only check if the models are in the environment.

3. Delete .on_load()

4. Add text to readme to explain that models should be explicitly loaded by calling load_models() while configuring the API.