I asked Claude, and now I know that there are nice, basically undetectable, plausibly deniable ways to render solar panels useless within months, or, with some preparation, days. It seems that open-source, carefully scrutinized panel controller firmware may be critically important.

Your Claude may have been hallucinating - solar panels have no microcontrollers or microprocessors, and hence have no need for firmware. Battery Charge controllers -- that are downstream of panels do have firmware, but even if a booby-trapped firmware were to short-circuit the panels, they'd be undamaged.