A 30-year-old transient pleaded guilty Friday and was immediately sentenced to 15 years in prison for slamming a skateboard over the head of a 79-year-old man in Costa Mesa. Jacob Riley Manndavis pleaded guilty to attempted murder, assault with a deadly weapon and inflicting injury on an elder adult, all felonies. He also admitted a sentencing enhancement for causing great bodily injury on the victim. Manndavis accepted a plea deal from Orange C…