For only $7$ stones, you can do it by hand. Define the positions as $P$(revious) and $N$(ext) depending on whether the previous player wins or the next player wins. A position is $N$ exactly when there is a move to a $P$ position. It is $P$ if there is no move to an $N$ position. Who wins a game with 1 stone? With 2 stones? With 3? Once you get to $7$ stones maybe you can see a pattern.