Add LiteLLM or llama.cpp support to Generate skills

[elcronos]

So far, only models like Anthropic or OpenAI are available for the generation of skills. I noticed that Eval has a way to load custom models and use them for evaluation. However, it doesn't seem possible to use custom models to generate skills. I'd like to have support to also add open source models or custom models to generate skills as oppose to proprietary only.

# Evaluate on local model (llama.cpp server)
upskill generate "parse YAML" \
    --eval-model "unsloth/GLM-4.7-Flash-GGUF:Q4_0" \
    --eval-base-url http://localhost:8080/v1

[sysradium]

#19
Basically if you set GENERIC_BASE_URL env var it'll work. My PR fixex that, such it is more obvious.