President Bush agreed yesterday to share civilian nuclear technology with India, reversing decades of U.S. policies designed to discourage countries from developing nuclear weapons. The agreement between Bush and Indian Prime Minister Manmohan Singh, which must win the approval of Congress, would create a major exception to the U.S. prohibition of nuclear assistance to any country that doesn’t accept international monitoring of all of its nuclear facilities. India has not signed the nuclear Non-Proliferation Treaty, which requires such oversight, and conducted its first nuclear detonation in 1974.