LONDON In a landmark collaboration, the U.K.’s domestic intelligence service MI5 has opened its vaults to the public for the first time, partnering with The National Archives to present an unprecedented exhibition on more than a century of covert operations. The result is “MI5: Official Secrets,” an exhibition that offers a rare look inside the inner workings of one of Britain ’s most secretive institutions, from World War-era spycraft to modern-day counterterrorism. The exhibit, now open at