Make default model for shelley.json user-configurable, or default to another model

Claude Opus 4.8 is (imo) overkill for setting up a VM. I would much prefer GLM or another model; much cheaper and just as good for this purpose.

Additionally, defaulting to Opus without a fallback means that users cannot really disable Anthropic in the llm integration; for me, there was no indication of what went wrong.