[codex] Mention llms.txt in agent docs

[phranzia]

Summary

• Add a quantiles.io/llms.txt reference to the Coding Agents section in README.md.
• Add the same concise LLM-readable overview pointer to AGENTS.md for coding assistants working in the repository.

Validation

• Reviewed the Markdown diff locally.
• No build or test suite run because this is a documentation-only change.