A Chilean judge on Monday formally charged former dictator Augusto Pinochet with homicide and kidnapping in one of many pending cases related to human rights abuses committed during his 17-year rule. ‘‘General Pinochet was declared mentally fit to stand trialin Chile,’’ Special Judge Juan Guzman told reporters. Pinochet’s defense had tried to argue he was not mentally competent to face the charges. Pinochet, who resides in a Santiago mansion and recently turned 89, could be placed under house arrest after formal notification of the charges, which usually takes a day. Pinochet’s attorneys plan to appeal Guzman’s decision.