For years, technologists have debated whether it is better for companies to keep the details of their computer code secret or share it with software developers around the world. That debate — closed versus open source — has become inflamed by the rapid development of artificial intelligence and worries that A.I. is quickly becoming a national security issue. In an open letter on Tuesday, Mark Zuckerberg, the chief executive of Meta, reinforced what some said was a risky stance taken by his company: that open source development of artificial intelligence would allow technologists to learn how powerful A.I. models are…