feat(llm): add per-provider interaction logging to file

[liut]

Add LogDir config to each Provider (openai/anthropic) for writing LLM interaction logs as JSONL files named by date and provider.

• Add LogDir field to settings.Provider struct
• Add logDir to llm.config and WithLogDir option
• Add audit.go with InteractionLog struct and LogInteraction func using mutex-protected open-write-close pattern (no file handle cache)
• Wire logging into openai.go Chat and StreamChat (parseStreamResponse)
• Wire logging into anthropic.go Chat and StreamChat (message_delta)
• Pass WithLogDir to all three llm clients in stores/llm.go