Function invocation 

pub fn invocation(
    profile: &HarnessProfile,
    lock: Option<&Lockfile>,
    lock_path: Option<&Path>,
    model: Option<&str>,
    prompt: &str,
    workspace: &Path,
    seed: u64,
    ctx: &Ctx,
) -> Result<Invocation, Error>

Build the invocation for a code harness. model is provider:model; when None it comes from the lock’s first LLM role.