Add local LLM backend feasibility analysis

Research and document the feasibility of adding local LLM support to
llm9p via an OpenAI-compatible backend. Analyzes the Backend interface,
surveys the local LLM ecosystem (Ollama, llama.cpp, vLLM, LocalAI,
LM Studio), proposes an implementation using sashabaranov/go-openai,
and estimates ~700 lines of new code following existing patterns.

https://claude.ai/code/session_017qVVZUUhfCCvNkMYmXDZAa