The New York Post (Marc Berman) reports:  Imagine how the course of Knicks’ history could have changed if Garden president Steve Mills and Magic Johnson had hooked up sooner — like in December 2003. Six years later, Mills is leaving the Garden to work with Johnson on various business ventures, MSG announced yesterday. A source said Mills will work on helping Johnson add boxing to his portfolio. Mills also will teach a course at his alma mater, Princeton, “Dilemmas in Athletics.” He found plenty of those in his nearly 10 years as a Garden executive — best known for hiring Isiah Thomas.