For decades, San Francisco has been blissfully removed from California’s water wars. The city’s pristine reservoirs in and around Yosemite National Park have been not only plentiful but also largely outside the reach of regulators. But plans by the state to mandate an increase in the amount of water flowing down rivers between the Sierra and San Francisco Bay — a bid to prevent the collapse of some of California’s most precious wetlands — has drawn the city into the fray.