Function run_command 

pub fn run_command(
    label: &str,
    program: &str,
    args: &[&str],
    output: &mut dyn Output,
    mode: OutputMode,
    viewport_size: usize,
) -> Result<CommandResult, ShellError>

Run a shell command with mode-appropriate output:

• quiet: collect output silently
• verbose: stream with | label... and > prefixed lines
• default: animated spinner overlay with scrolling viewport