My friend Pam Harbin is trying to undergo a startling metamorphosis.     She wants to transform from an education activist into a Pittsburgh School Director.    Now that Board President Lynda Wrenn is stepping down after 4 years, city voters in District 4 will have to decide whether Harbin can make the change. The election is on May 21.     Residents in parts of Squirrel Hill, Point Breeze, Shadyside and North Oakland already know Harbin as a…