A former career US diplomat told a federal judge on Thursday he will plead guilty to charges of working for decades as a secret agent for communist Cuba, an unexpectedly swift resolution to a case prosecutors called one of the most brazen betrayals in the history of the US foreign service.Manuel Rocha’s stunning fall from grace could culminate in a lengthy prison term after the 73-year-old said he would admit to federal counts of conspiring to act as an agent of a foreign government.Prosecutors and Rocha’s attorney indicated the plea deal includes an agreed-upon sentence, but they did…