This is a community plugin and is thus not built-in to proto. In order to use it, first either add it to your global or project-based .prototools by running:
proto plugin add grain "source:https://raw.githubusercontent.com/Phault/proto-toml-plugins/main/grain/plugin.toml" --global
proto install grainproto plugin add grain "source:https://raw.githubusercontent.com/Phault/proto-toml-plugins/main/grain/plugin.toml"
proto pin grain latest --resolve