Function run_exec 

pub async fn run_exec(
    selection: HostSelection,
    command: &str,
    config_override: Option<PathBuf>,
    format: OutputFormat,
    json: bool,
    opts: ExecOptions,
) -> Result<()>

Runs a shell command on one VPS or a multi-host selection (bounded).

Workload: I/O-bound SSH. Multi-host (All / Named) uses crate::concurrency::map_bounded. Batch JSON when HostSelection::is_batch.