Function parse_model_arg 

pub fn parse_model_arg(
    arg: &str,
    harness: &str,
    model_config: &ModelConfig,
) -> Option<(Tier, String)>

Parse a /model <arg> argument: either a tier name (low|med|high) or a concrete model name (opus|sonnet|...).

Returns the resolved Tier and the concrete model name (the original arg if it was already a concrete name; the resolved name from config/built-ins if it was a tier name).