HARARE (Reuters) - Zimbabwe President Robert Mugabe has agreed to stand down and his resignation letter has been drafted, CNN said on Monday, citing a source familiar with his negotiations with the generals who seized power in Harare last week. Under the terms of the deal, Mugabe and his wife Grace would be granted full immunity, CNN said. Two senior government sources told Reuters late on Sunday that Mugabe had agreed to resign but did not know details of his departure. 