Under a landmark agreement announced yesterday, the three largest grocery chains in Southern California will buy 150 alternative-fuel big rigs to replace heavily polluting diesel trucks. The deal settles a lawsuit filed in 1998 by enviro groups and the state attorney general’s office, which alleged that the companies were exposing people to hazardous diesel exhaust without issuing warnings as required by law. California Attorney General Bill Lockyer said the agreement could prod other companies to move away from diesel fuel, the source of 70 percent of California’s toxic air pollution. The deal is also expected to provide a high-profile boost to alternative-fuel truck technologies that have not yet come into wide use.