A $2.4 billion agreement with the Lehman Brothers bankruptcy over the valuation of mortgage-backed securities is the latest in a string of such pacts negotiated by Houston-based Gibbs & Bruns. The deal takes the massive bankruptcy, now eight-and-a-half years old and counting, one small step toward resolution. It also burnishes the reputation of the Houston litigation boutique as a go-to firm for mortgage-backed securities claims. Here are details of the latest deal.