I walked into Lefty’s last Wednesday and followed the musical advice coming from the stage. I knock, knock, knocked on wood, baby. And bartender Mike Ziimerman bought me a pint and a menu. But I didn’t need any luck, because I knew with Mary Buck singing and some great players backing her up, a killer musical night was already assured. Mary has been hosting Open Mic nights at Lefty’s on Wednesdays for the past two and a half years with musicians coming from all over to join in the fun and jams. On this night, a young lady on vacation from Pittsburgh bravely faced an unknown crowd. Starting …