Richard Hell — writer of the de facto theme song to the CBGB’s ’70s scene, “Blank Generation” — says he knew punk was done when its edgy look hit department stores. “I had mixed feelings when that happened,” Hell, whose new memoir about the era is called, “I Dreamed I Was a Very Clean Tramp,” told us last week. “Partly it was gratifying because we had an impact on the wider world, but [its] showing up in Macy’s windows finally meant it was time to do something new . . . it was over.” Leaving the safety pins behind and now wearing a suit, he added, “I don’t really associate myself with it anymore.” But his iconic look will be part of the Met Museum’s upcoming “Punk: Chaos to Couture.”