Function resolve_env_file 

pub fn resolve_env_file(
    explicit: Option<&Path>,
    no_env_file: bool,
    dir: &Path,
) -> CliResult<Option<PathBuf>>

Resolve the .env file to load (if any). Precedence:

1. If no_env_file is set, return Ok(None).
2. If explicit is Some(path), the file must exist — otherwise error.
3. Otherwise, return Some(dir/.env) if it exists, else None.