Function handle_shell_status 

pub(super) async fn handle_shell_status(
    __arg0: State<AppState>,
    __arg1: Json<ShellStatusRequest>,
) -> impl IntoResponse

POST /api/v1/shell/status — query a shell’s running state (Phase 3b).

Returns { running, exit_code, line_count }. If the shell_id is unknown (never started or never seen by this sidecar), running is false and exit_code is absent.