NITA BHALLA IT'S hard to keep up with Renee Ngamau.  From mentoring women entrepreneurs to campaigning against police brutality, the Kenyan lawyer, businesswomen, human rights activist - and former prime-time radio disc jockey - has taken up many social causes over the years. But Ngamau's latest crusade to protect Kenya's urban playgrounds has perhaps hit home more than most as she finds herself fighting to save her own housing estate from losing its green spaces to illegal encroachment by private developers. "I grew up here. I used to swing from the branches of that tree right over there. We used…