WALNUT CREEK, California – Etched in dirt, a narrow furrow is the only clue that the grasslands of Lime Ridge Open Space will soon be restored to their original splendor, cleared of dangerous power lines that could ignite nearby subdivisions. The undergrounding project, costing $3.75 million a mile, represents the beginning of a 10,000-mile-long effort by Pacific Gas and Electric to bury the state’s distribution lines to cope with the growing risk of winds and wildfires linked to global warming. “It is a one-time investment to eliminate essentially all ignition risk related to power lines, wit…