Allow ilab model chat to accept a system prompt #2978

The ilab chat command today provides an unrivaled chat experience at the CLI. However; it's missing some critical features which are commonplace with other AI chat applications.
Namely, the ability to specify your own system prompt to the model.

You should be able to override the default system prompt by specifying .chat.system_prompt in the ilab config, or you can pass a --system-prompt flag to ilab chat.