It was 25 years ago tonight when thieves masquerading as Boston police officers entered the Isabella Stewart Gardner Museum and pulled off the biggest art heist in American history. They stole 13 works now valued at more than $500 million. In the interim years, the museum’s director, Anne Hawley, has not spoken a lot about the case. But she recently sat down with WGBH Arts Editor Jared Bowen for her only TV interview on the subject.