HAVANA — Fidel Castro developed a leadership model few management schools would endorse as a healthy approach to institutional robustness. Government ministers and other top figures tended to remain in power until brought down by illness, error or scandal. Retirement was a kind of capitulation, a sign of weakness. So President Raul Castro’s declaration that he wants the government to implement term limits for top posts was a clear and relatively radical break with his older brother’s iron-man leadership philosophy.