Use $EDITOR as default editor if set

Is your feature request related to a problem? Please describe. I've set $EDITOR to helix. Editing a role in REPL mode still opens nano instead.

Describe the solution you'd like aichat uses $EDITOR (if not empty), if nothing is configured in config.yaml

Describe alternatives you've considered As a workaround, I've set editor: helix manually in config.yaml.