/**
 * Return the current ISO day of the week as a number in the system timeZone.
 *
 * - Uses Temporal.Now.zonedDateTimeISO to get current day in system timezone.
 * - Returns ISO day of week: 1 (Monday) to 7 (Sunday).
 * - Returns null when system timezone is unavailable.
 *
 * @returns current ISO day of week number (1-7) or null on invalid
 *
 * @example getDayOfWeek() // 4
 * @example getDayOfWeek() // null (when system timeZone unavailable)
 */
export declare function getDayOfWeek(): number | null;