Before becoming a prosecutor this year, Robert Steele's career as a defense attorney spanned 24 years and more than 100 trials. Now, in his very first trial for the St. Louis Circuit Attorney's Office, Steele is arguing that St. Louis cop Jason Stockley committed premeditated murder and planted a stubby .38 revolver in the car of his victim.No video clearly shows Stockley slipping the gun into the wrecked Buick driven by Anthony Lamar Smith on December 20, 2011.…