n an abrupt shift in teacher contract negotiations, Superintendent Carol R. Johnson retreated Wednesday from a sweeping proposal to add 45 minutes to Boston’s elementary, middle and K-8 schools, and instead will pursue a narrower effort that would add two hours at a handful of low-performing schools. Extending the school day had been a hallmark of Johnson’s push to secure substantial changes in the contract in an effort to reinvigorate schools across the city. But after two years of intense and exhaustive talks with the teachers union over a new contract, Johnson said it appeared her proposal was going nowhere as the union insisted its teachers be paid at a rate that she said the city cannot afford, prompting her to pull the 45-minute proposal.