The popular Boston-area coffee chain Darwin’s Ltd. announced plans to close the store’s original Harvard Square location at the end of the month, prompting some workers to stage a protest at Cambridge City Hall on Sunday denouncing the move. The Mount Auburn St. location is set to close its doors after 30 years this December, owners Steven and Isabel Darwin announced in an Instagram post on Oct. 26. Darwin’s United — a union representing the chain’s employees — responded by organizing a protest at City Hall, where workers rallied on Sunday before gathering outside the Darwins’ Cambridge home. “We have...