HOUSTON — The skyscrapers at the Texas Medical Center are filled with hospital beds. The commuters waiting in the heat for the train wear scrubs. The young woman dodging traffic on a bicycle may be studying to be a doctor, a nurse or a dentist. Houston is known for its energy jobs, but registered nurses are the most in-demand workers in town. “Whatever jobs we have, we keep expanding. And our competitors are doing the same,” said Thomas Vernon, a human resources director for Houston Methodist, which has a large hospital at the medical complex here. Last year the health system hired about 1,200 nurses across its seven hospitals.