The CIA should have known something was off the moment ex-officer David Rush demanded 667 pounds of gold bullion. Instead of requesting diamonds or rare gems — the far lighter, more mobile currency the agency has long favored for clandestine work — Rush asked for heavy gold bars, a choice a former senior CIA official told The Post was an obvious red flag. Now Rush is incarcerated in an Alexandria, Va., lockup and charged carrying out a massive theft – although it is the spy agency itself facing