HELENA, Mont.—A Montana prison inmate serving a 100-year sentence for killing his wife in 1983 has confessed to strangling, sexually assaulting, and dismembering a 19-year-old Helena woman 15 years earlier, authorities said Friday. Courtney Brooke Atlas, 79, confessed to killing Pamela Ann Dorrington, who disappeared in 1968. Part of her torso was found near the…