Add AGENTS.md with repo structure and build guidance

[drbenvincent]

Summary

• Adds an AGENTS.md file documenting repo conventions for AI coding agents
• Covers notebook file pairing (.ipynb source of truth, .myst.md auto-generated)
• Documents that RTD builds don't execute notebooks, so missing Python packages can't cause RTD failures
• Notes that RTD "Unknown problem" failures with short duration are transient and should be retriggered
• Reminds agents not to commit scratch artifacts

Test plan

• Pre-commit hooks pass
• Verify file renders correctly on GitHub

Made with Cursor