BOSTON - The field of potential Democratic candidates for the 2006 governor's race is already shrinking. U.S. Rep. Michael Capuano [related, bio] said Friday he's decided not to pursue the party's nomination for the corner office, saying he wants to focus on working in Washington on issues like Social Security, affordable housing and lobbying for transportation dollars for Massachusetts. ``After much personal deliberation with my family and those closest to me, I have decided that I will not be a candidate for governor,'' Capuano said in a statement. ``As much as I am attracted to the campaign and the possibility...