A former Intel engineer has been charged with stealing trade secrets worth $1 billion from the chip maker while he worked for its main rival, Advanced Micro Devices. Federal prosecutors in Massachusetts alleged this week in a five-count indictment that Biswamohan Pani, 33, illegally downloaded more than a dozen confidential documents from Intel’s computer system in California during a four-day stretch in June. He had already resigned from Santa Clara, Calif.-based Intel, but remained on the payroll and still had access to the company’s computers while he burned unused vacation days.