By Humeyra Pamuk and Doina Chiacu WASHINGTON (Reuters) – U.S. ex-Marine Trevor Reed arrived back in the United States, his spokesperson said on Thursday, after being freed by Russia in a prisoner swap that took place amid the most tense bilateral relations in decades over the war in Ukraine. Reed was released on Wednesday in exchange for Russian pilot Konstantin Yaroshenko. The swap was not part of broader diplomatic talks and did not represent an American change in approach on Ukraine, U.S. officials said. Russian-American ties have been at their worst since the Cold War era following Russia’…