Enable fixture AI agent writes via DataAccessor

[xziy]

Summary

• extend the fixture OpenAI data agent with a create_model_record tool that writes through DataAccessor
• update the fixture configuration and documentation to reference the single openai-data assistant model
• record the new workflow in HISTORY.md and describe the JSON command envelopes for reads and writes

Testing

• npm run lint (fails: Missing script "lint")

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https://chatgpt.com/codex/tasks/task_e_68d776dec6e0832a8d33224c09bde433