NIAMEY, NIGER — A major journalist in Niger was in custody Saturday, an association said, a day after the private TV channel he runs was suspended following a report critical of the military-led regime. Seyni Amadou, editor in chief of Canal 4 TV, has been arrested, said CAP-Medias-Niger, which represents media workers in the country. On Friday, Niger’s communications ministry announced the channel had been taken off the air for a month. State television channel Tele Sahel said Canal 3 TV had been punished “for violating the rules of ethics and deontology.” Canal 3 told AFP in a statement that