A South San Francisco man accused of throwing a punch that resulted in the death of former Reagan Administration adviser David Winston, 45, pleaded no contest to involuntary manslaughter. Andrew B. Heagney, 22, entered the plea in San Francisco Superior Court on an agreement approved by Winston's family. Under the agreement, Heagney will be sentenced to three years' probation and six months in jail. But he might qualify for a work furlough program under which he would be freed to continue employment as a sheet metal worker while spending his nights in jail. Winston, who advised the Reagan Administration on health care matters, died Sept. 6 when he struck his head on a curb after being knocked down by Heagney in a traffic dispute.