DALLAS — A Flowery Branch native and former actress pleaded guilty Tuesday to sending ricin-laced letters to President Barack Obama and New York Mayor Michael Bloomberg, under a deal that her attorney has said would cap prison time at 18 years. Shannon Guess Richardson entered her plea in federal court in Texarkana, Texas, to a federal charge of possessing and producing a biological toxin. Richardson was arrested in June after authorities said she tried to implicate her estranged husband, Nathan Richardson, after he had filed for divorce.