Imagine that the Central Intelligence Agency considered influencing world events by sending a world leader poisoned socks, making his hair fall out or having his picture taken carousing with babes while eating forbidden foods. Now imagine that all the above is true. Which it is. Playwright Brian Stewart?s reaction to these revelations about CIA plans to eliminate Fidel Castro was to keep on laughing. The felicitous result is Castro?s Beard, which has its New England premiere at Barrington Stage Company.