In a landmark vote closely watched across California, Silicon Valley’s largest water agency on Tuesday rejected Gov. Jerry Brown’s $17 billion plan to build two giant tunnels under the Sacramento-San Joaquin River Delta. By a vote of 7-0, the Santa Clara Valley Water District’s board of directors chose instead to adopt a set of principles endorsing a significantly smaller, less costly project — with just one tunnel. “What magnitude of project makes sense?” asked board member Barbara Keegan. “From our perspective that looks to be something less than originally was envisioned.”