Add AGENTS.md

[WeiqunZhang]

Written by Codex.

[atmyers]

Should we include 1) a short description of AMReX, and 2) an overview of the source layout, as the WarpX one does?

[WeiqunZhang]

Codex says,

AGENTS.md exists to give “agentic” tools enough repo-specific context to work safely: what branch to target, how to build/test, where docs live, how to log handoffs, and the quirks (copy/paste bugs, local issues folder). Everything inside should help an automated reviewer or implementer spend less time rediscovering conventions.

Adding a short AMReX description probably isn’t necessary unless it conveys unique expectations (e.g., “this repo is a block-structured AMR framework with C++/Fortran, multi-backend GPUs”). Agents can already read the top of README.md if they need a product summary, and duplicating it risks drift.

A high-level source layout overview, however, tends to be more useful because it saves a lot of navigation (“physics kernels live in Src/AMRCore, drivers under Tests/, docs under Docs/sphinx_documentation, etc.”). That’s the kind of orienting info agents can’t infer instantly from README, yet it keeps responses grounded and reduces mistaken edits. I’d keep AGENTS.md focused on actionable guidance like that—anything that short-circuits tedious discovery is worth the context cost; generic marketing blurbs are not.

[WeiqunZhang]

Add a section on source layout.

[WeiqunZhang]

added a short description.

[atmyers]

Claude recommends the @ file include syntax over a soft link for Windows portability.