Add an AGENTS.md authoring guide #20

Summary

Add a guide on authoring AGENTS.md — the cross-tool standard for giving AI coding agents project-specific instructions. Our colleagues run several agents (GitHub Copilot CLI, OpenCode, Pi), and a single AGENTS.md is read natively by all of them, so conventions are written once instead of re-explained every session (saving input tokens and forming a stable, cacheable context prefix).

The guide should cover

• What AGENTS.md is and why it matters for a multi-tool, budget-conscious team.
• What to put in it (and what not to).
• Nested per-language AGENTS.md for our stack — Python, Node, Scala, .NET — since conventions differ per language.
• How it relates to tool-specific files (e.g. copilot-instructions.md, CLAUDE.md) and which to prefer.
• A short starter template.

Notes

• Back the findings with web research across multiple independent sources.
• Keep it tool-general (team primarily uses GitHub Copilot CLI, plus OpenCode and Pi).