Welcome! I don't think there are any rules like what you're looking for, except for what is allowed by Robocode itself. Reflection, network access, and general filesystem access are all disabled by Robocode's security manager, and your CPU time is limited to a few milliseconds per turn. You do have a small amount of controlled disk access - see AdvancedRobot.getDataFile. As for CPU time, if you use more than the amount Robocode calibrated for your system, your bot will skip some turns to make up for it.
Hope that helps and let us know if you still have questions.