Function cmd_show 

pub fn cmd_show(
    id: &str,
    json: bool,
    short: bool,
    history: bool,
    beans_dir: &Path,
) -> Result<()>

Handle bn show <id> command

• Default: render beautifully with metadata header and markdown formatting
• –json: deserialize and re-serialize as JSON
• –short: one-line summary “{id}. {title} [{status}]”
• –history: show all history entries (default: last 10)