An NYPD traffic agent pleaded guilty Tuesday in the drunken high-speed chase that killed a Brooklyn woman on her birthday. Stefan Hoyte, 26, was driving his 2013 Infiniti G37 at 111 mph and his blood-alcohol level was .12 — more than the legal limit of .08 — when he smashed into a pillar on the Williamsburg Bridge on March 16. His passenger,…