The Supreme Court signaled Monday a readiness to upend a 90-year-old precedent and grant presidents expansive powers to fire top officers at independent agencies, saying the way the commissions and boards are run now insulates them from political accountability. Chief Justice John G. Roberts Jr. declared the 1935 precedent, a case known as Humphrey's Executor, to be "just a dried husk" that has been overtaken by law and the modern expansive administrative state that runs much of government. He and the court's other Republican-appointed justices seemed ready to take up the call by President Trump's attorney, Solicitor General D. John Sauer, to find that agencies such as the Federal Trade Commission, the National Labor Relations Board and the Merit Systems Protection Board must fall under full presidential control.