Store API keys in OS keychain system

Description

Currently the API tokens for the LLMs are stored in ~/.local/share/crush/crush.json. Storing these API keys in plain text is not ideal.

On different operating systems there are a concept of a 'keychain' where the information should be stored instead. In fact, VSCode and Zed both use this on macos (I verified this by hand).

https://github.com/zalando/go-keyring is a potential keyring package that has only 2 dependencies (dbus and wincred)