DirecTV reached a $12 billion agreement to continue airing National Football League games on Sundays, removing a potential obstacle to the company's purchase by AT&T Inc. The eight-year deal for NFL Sunday Ticket will cost DirecTV an average of about $1.5 billion annually, according to a person with knowledge of the matter who asked not to be identified because the terms are private. The price underscores the high value the TV industry places on live sports, especially the NFL.