feat: allow API base URL override via flag or env var

[outofcoffee]

Override the provider API base URL for any provider via a command-line flag or environment variable.

Summary

• Add --base-url/-b flag to oc-config add to set options.baseURL for any provider
• Add OC_CONFIG_BASE_URL env var as an alternative override
• Override applies to every provider, including those with no base URL in the catalogue, and wins over the catalogue's static and per-provider (OLLAMA_BASE_URL, etc.) base URLs
• Add tests covering the flag, the env var, their precedence, and end-to-end config writing
• Document the new override in the README and the llama.cpp guides

Implementation details

Precedence is resolved as flag → OC_CONFIG_BASE_URL → catalogue defaults. The flag value is threaded through buildProviderBlock, which falls back to the env var (via the existing resolve function) when the flag is empty, then sets options.baseURL last so it overrides both the static options.baseURL and any per-provider optionsFromEnv mapping.