Lady Bird Johnson, the former first lady who championed conservation and worked tenaciously for the political career of her husband, Lyndon B. Johnson, died Wednesday, a family spokeswoman said. She was 94. Johnson, who suffered a stroke in 2002 that affected her ability to speak, returned home late last month after a week at Seton Medical Center, where she had been admitted for a low-grade fever. She died Wednesday afternoon at her Austin home of natural causes. Elizabeth Christian, the spokeswoman, said she was surrounded by family and friends.