The boyfriend of a Bronx mom fatally shot in broad daylight says he believes his girlfriend knew her killer.Police say Kenia Castillo-Reyes was shot in the head on the sidewalk in front of her building at 710 Coster St. around 3 p.m. yesterday.The 43-year-old mother of two was likely killed by someone she knew, her boyfriend says. But he says he doesn't know who took her life.Anyone with information on the murder is urged to call Crime Stoppers at 1-800-577-TIPS.Woman dies after Hunts Point shooting