Russia’s oil-market war with Saudi Arabia is part of a strategic campaign to cripple U.S. shale-oil production, a powerful economic tool that increasingly allows Washington to advance its foreign policy agenda, say people briefed on the Kremlin’s policies. Less than two weeks ago, President Vladimir Putin summoned Russian oil companies to a conference room at Moscow’s Vnukovo airport to discuss strategy ahead of a meeting between OPEC and its allies on March 5-6. …