Nobody can say the San Francisco Chronicle isn’t doing everything in its power to promote “Zodiac,” the newly-opened thriller that centers on efforts by one of its star reporters and an editorial cartoonist there to crack the famous serial-killer murders of the ’60s and ’70s. For today’s Chronicle, the Hearstites have dug into the archives and come up with this previously unpublished 1990 greeting card that a handwriting expert says may have been sent by The Zodiac.