[codex] add Diataxis guidance to agents

[odyslam]

Summary

Adds repo-specific Diataxis guidance to AGENTS.md so documentation work in this repo has explicit rules for:

• classifying pages by Diataxis type
• editing incrementally instead of forcing top-down reorganization
• organizing pages, titles, openings, and landing pages by doc type
• using the right language and voice for tutorials, how-to guides, reference, and explanation
• reviewing pages for mixed-mode blur and reader flow

Why

The docs are built with Diataxis, but the local agent guidance only covered Mintlify and general writing constraints. This change makes the Diataxis expectations operational for future doc edits in this repo.

Impact

• Improves consistency of future documentation changes
• Reduces the chance of mixed tutorial/how-to/reference/explanation pages
• Gives agents a clearer standard for navigation and landing-page organization

Validation

• Reviewed the upstream Diataxis source material and translated the relevant authoring rules into repo guidance
• No automated checks were applicable for this AGENTS.md policy-only change