WASHINGTON — “Once an agent, always an agent.” This was the terse response of Nina Khrushcheva on New Year’s Eve 1999 when her mother commented favorably about the new president, Vladimir Putin, who was then speaking on TV. Khrushcheva, great-granddaughter of former Premier Nikita Khrushchev, was prescient then and feels no need to revise those comments now. Instead, her mother’s early reviews were symptomatic of what Khrushcheva calls the “gulag of the Russian mind,” part of the title of her just-published book, “The Lost Khrushchev: A Journey into the Gulag of the Russian Mind.”