Function run_health_check 

pub async fn run_health_check(
    selection: HostSelection,
    config_override: Option<PathBuf>,
    format: OutputFormat,
    json_local: bool,
    password_override: Option<SecretString>,
    timeout_override: Option<TimeoutMs>,
    key_override: Option<String>,
    key_passphrase_override: Option<SecretString>,
    replace_host_key: bool,
) -> Result<()>

Health-check SSH (single host or multi-host bounded fan-out).

Workload: I/O-bound connect probe. One-shot auth parity (GAP-SSH-CLI-006) and TOFU (M1). Multi-host saturates sockets/auth — gated by concurrency budget. Batch JSON when HostSelection::is_batch (G-PAR-36).