fix(index): honor llamacpp provider and show underlying error details

[randomshinichi]

Unrelated: you have a mac library committed in there, lib/libcozo_c.a

This PR fixes two issues in cie index:

1. embedding.provider: llamacpp was not honored by CLI mapping
   • cmd/cie/index.go only mapped a limited set of providers and silently fell back to mock for unknown values.
   • As a result, valid llamacpp config could be downgraded to mock at runtime.
2. Underlying error details were hidden in CLI output
   • internal/errors.UserError.Format() printed only high-level message/cause/fix.
   • Wrapped/root errors were not shown, making troubleshooting difficult.

What changed

cmd/cie/index.go

• Updated provider mapping to explicitly support:
  • llamacpp
  • qodo
• Changed default behavior to preserve unknown provider strings so provider validation happens in the embedding layer (instead of
  silent fallback to mock).
• Added llama.cpp env wiring:
  • LLAMACPP_EMBED_URL now populated from config embedding.base_url.

internal/errors/errors.go

• UserError.Format() now appends:
  • Details:
• This surfaces wrapped exceptions directly in CLI output.