Pro baseball becomes real for Mark Appel on Friday. The No. 1 overall pick of the 2013 MLB amateur draft is scheduled to make his debut for the short-season Tri-City ValleyCats against the Lowell Spinners in Troy, N.Y. Appel will be on a pitch count Friday and is on an overall minor-league affiliate schedule this season, Astros general manager Jeff Luhnow said Tuesday at Minute Maid Park. “He threw a side (session in Kissimmee, Fla.) and we wanted to give him a day off. … Tri-City is going to promote it and hopefully fans will come out and see him,” Luhnow said. The former Stanford righthander is expected to soon [.]