Assuming you're pretty sure you're going to do the YC thing either way, the right answer is: renege on the summer offer. It's at-will employment, and your will

Assuming you're pretty sure you're going to do the YC thing either way, the right answer is: renege on the summer offer. It's at-will employment, and your will is not to work there. Apologize nicely, and tell them you hope for future opportunities at their company. Internships are just expensive recruiting anyway.