DETROIT — After three decades at work in a GM factory, John Martinez has reached a crossroads. Martinez, 50, must choose between retiring and making a long and expensive commute across state lines to stay with General Motors Corp. Any future he can imagine is going to be costly and tough. ‘My whole family is under stress,’ he said. The same can be said of the embattled U.S. auto industry and its recession-hardened hometown, Detroit. GM, once an emblem of U.S. post-war economic might, is being driven to the brink by dwindling sales that are expected to test cash reserves and the nerves of investors in the months ahead. Crosstown rivals Ford Motor Co and privately held Chrysler LLC face similar pressures. As the automakers weigh their options to ride out the industry’s most-trying slump in 25 years, thousands of Detroit families are doing the same. For many, the choices line up from bad to worse. With four kids, retirement is not an option for Martinez. But driving more than 100 miles daily between home in the Detroit suburb of Lincoln Park and Toledo, Ohio — where GM has a job for him — […]