Add OpenAI-compatible backend for local LLM support (GPT-OSS)

Implement OpenAIClient backend that speaks the OpenAI Chat Completions
API (/v1/chat/completions), enabling llm9p to work with any local model
server: Ollama, vLLM, llama-server, LocalAI, or LM Studio.

Primary target is GPT-OSS (OpenAI's open-weight MoE models), but any
model served via these platforms works. The backend supports:
- Blocking and streaming chat completions
- Tool/function calling with STOP:/TOOL: formatting
- Token counting from API usage (with estimation fallback)
- Conversation history, system prompts, temperature control
- Stateless AskWithRequest for session isolation

Usage:
  ./llm9p -backend openai -openai-url http://localhost:11434/v1 -model gpt-oss:20b

Also fixes pre-existing stale mock backends in test files (AskWithRequest
signature was out of date with the Backend interface).

https://claude.ai/code/session_017qVVZUUhfCCvNkMYmXDZAa

feat: Initial implementation of llm9p - LLM as 9P filesystem

Exposes Claude as a 9P filesystem, enabling interaction through
standard file operations:

- ask: write prompt, read response (shim pattern)
- model: read/write current model name
- temperature: read/write sampling temperature
- tokens: read-only token count from last response
- new: write to reset conversation
- context: read JSON history, write to add system message
- _example: usage documentation
- stream/chunk: blocking read for streaming responses

Includes:
- Full 9P2000 protocol implementation (stdlib only)
- Anthropic SDK integration with conversation state
- Streaming support

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>