Function load_with 

pub fn load_with<F>(
    agent_dir: &Path,
    cli: &CliOverrides,
    env_lookup: F,
) -> Result<Settings>
where
    F: Fn(&str) -> Option<String>,

Load Settings, overlaying file (if present) → env → CLI.

env_lookup is a closure so tests can inject deterministic env values without mutating the process environment.