Add internal/llamacpp package with Client type and Health() method.
Client communicates with llama-server via HTTP; Health checks the
/health endpoint and reports readiness. Foundation type for the
streaming methods (Tasks 2-3).
Co-Authored-By: Virgil <virgil@lethean.io>
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
