There are certain things about Sept. 11, 2001, that remain as clear to me today as they were 25 years ago. One of them is the sky. It was a beautiful, crystal-blue September morning, the kind of day we welcome as summer begins to give way to fall. Yet for me, and for so many others, a clear blue September sky has never looked quite the same since. Even today, when I walk outside on a morning with that same crisp air and brilliant blue sky, I’m i…