Turn off HighlightJS autodetect language

in particular, copy-pasting error messages currently looks significantly worse than nothing

<img width="820" height="163" alt="Image" src="https://github.com/user-attachments/assets/f73b9156-68f5-4fd3-b7cf-27528dec1f9b" />

our goal is to match the preview as best we can, and I believe GitHub does not guess languages (TODO: confirm).