Function select_one 

pub fn select_one(
    prompt: &str,
    options: &[String],
    is_json: bool,
) -> Result<usize, SelectError>

Prompt the user to select one option from a list using dialoguer.

• is_json: return Err(SelectError::JsonModeNeedsExplicit).
• Non-TTY: auto-select first option with stderr warning.
• Interactive: use dialoguer::Select on stderr.