The Cardinals will have to hit a whole lot better this summer to hang in the playoff chase. That’s why long-struggling shortstop Paul DeJong is heading back to the minors for remedial work. That’s why Edmundo Sosa will get a “real shot” to play shortstop, as manager Oliver Marmol put it, once he returns from his rehabilitation assignment at Double-A Springfield. That’s why ascending hitter Juan Yepez was back in the lineup Tuesday, hitting cleanup as the designated hitter. That’s why elite prospect Nolan Gorman will get his first big league at-bats at some point this season. During a recent 10…