A 40-year-old man has a discolored maxillary lateral incisor that was traumatized 10 years ago. The tooth does not respond to cold or electric testing, and a 12-mm well-defined radiolucency surrounds its apex. The adjacent central incisor responds normally, and the incisive canal appears separate from the lesion. What is the most likely origin of the lesion's epithelial lining, and the first line of treatment?