PHILADELPHIA — The gates to the Eagles’ practice fields swung open at the Jefferson Health Training Complex on Friday afternoon, and the only “action” on the field closest to reporters was rookie Cole Payton being coached up by quarterbacks coach Parks Frazier. Overseeing the drills more broadly was a lanky 6-foot-6 figure catching some of Payton’s throws while offering his own pointers and encouragement. It was new Eagles offensive coordinator Sean Mannion. Mannion, who turned 34 on April 25, has yet to be formally introduced as the team’s offensive coordinator. That’s not unusual in the modern Eagles organization, which has increasingly delayed such announcements until on-field OTAs begin. Even veterans like Vic Fangio and Kellen Moore were kept under wraps until then. The last coordinator introduced in a timely fashion was Jim Schwartz during the Doug Pederson era.…