Add an AGENTS.md authoring guide

## 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).