BAGHDAD, April 4 (Reuters) - The U.S.-led coalition in Iraq confirmed on Sunday it had detained Mustapha Yacoubi, an aide to radical Shi'ite cleric Moqtada al-Sadr, in connection with the killing of Shi'ite cleric Abdul Majid al-Khoei last year. Yacoubi's detention stoked anti-American protests by Sadr supporters across the country on Sunday. The largest was near Najaf, where at least 24 people were killed when angry Shi'ite protesters clashed with coalition forces and Iraqi police. Khoei was hacked to death at a Najaf mosque in April, 2003 by a mob which also killed one of his aides. Senior clerics at...