“Dead man walking” are the words used in many prisons to announce a death-row prisoner is being led to the execution chamber.It’s also the title of the 1995 film, starring Susan Sarandon and Sean Penn, adapted from Sister Helen Prejean’s account of her first experience as spiritual adviser to a convicted murderer, whom she accompanies to his execution.In 2000, this true story, with a libretto by famed playwright Terrence McNally and music by Jake Heggie, made its premier as an opera in San Francisco …