EDIT: figured it out. My Zed settings was broken, because it tries to create a settings file in "~/.config/zed", and my entire "~/.config" directory is symlinked somewhere else, so macOS permissions broke Zed's ability to create that config file. So I gave Zed full-disk access in the macOS "Privacy & Security" settings, Zed is now able to make a config file (which is where the model is set when you choose one), and everything is hunky dory.