San Francisco officials have reached an important milestone in a long-running effort to build a high-pressure water network needed to bring vital firefighting capabilities to the Richmond and Sunset districts — two neighborhoods that have historically lacked direct access to such a system. After years of planning and deliberations, the San Francisco Public Utilities Commission and the city’s Fire Department have finally picked a design for a new auxiliary water supply system that can supply firefighters with large volumes of high-pressure water needed to put out rampant blazes that could result from earthquakes or other major emergencies. And in what city water and fire officials say is an added benefit, the proposed 9.75-mile-long system will also be built to carry drinking water, helping to ensure that more potable water is available after an earthquake.