Tricia McLaughlin, a spokeswoman for the Department of Homeland Security known for her combative style and forceful defense of President Trump’s immigration agenda, is stepping down from the agency, according to two officials with knowledge of the decision. Her planned departure comes after a tumultuous month for D.H.S. in which three people were shot by immigration officers in Minneapolis; two died, and department officials have walked back their claims about the third, whose injuries were nonfatal. Ms. McLaughlin had made plans to leave the agency in December but extended her service after the first two shootings, one homeland security official...