GENEVA (Reuters) - U.N. Syria envoy Staffan de Mistura said Syria talks in the Kazakh capital Astana on Friday had agreed on a  working group  for the release of detainees, which he said was a commendable first step towards an arrangement between the warring sides.  He also said that Russia s plan to convene a Syrian  congress of national dialogue  in Sochi next month should be assessed by its ability to contribute to and support the U.N.-led Geneva talks on ending the war in Syria.  