LUCILA SIGAL  AN 83-year-old widower on a mission to investigate potential abuse at a nursing home reveals telling lessons about the trials of growing old in Chilean filmmaker Maite Alberdi's Oscar-nominated documentary "The Mole Agent." Although the story rings like the plot of a fiction film, the drama that unfolds is real. The tender and touching feature film, nominated for best documentary, is the only Latin American nominated for an Academy Award. With equal doses of humor and film noir, the 90-minute documentary tells the story of Sergio Chamy, a confounded but enthusiastic sleuth who barely knows how to handle…