Michael Corbat looked like a safe pair of hands when he took over from Vikram Pandit as Citi’s chief executive. Known as a details man, a leader and strong manager, he will execute the strategy Pandit first laid out. But overseeing the rundown of Citi Holdings has given him a taste for better deploying scarce resources. And every business in Citi’s renowned 100-country network, the bank’s defining differentiator, will have to justify itself to his scrutiny.