Restrict which models are available in `/model`

What variant of Codex are you using?

CLI

What feature would you like to see?

We want to be able to configure the model list visible to users when they run /model.

Additional information

In an enterprise environment where models are served from Azure, not all models, especially the latest models, are enabled for users. As a result, user may get errors if they switch to a model (with /model/) that is not yet supported. We want to restrict the list to only supported/approved models. This kind of "restrict model selection" is available in other coding agents and working well for us.