Initial AGENTS.md

I've been using agents a lot more with Ghostty and so are contributors.
Ghostty welcomes AI contributions (but they must be disclosed as AI
assisted), and this AGENTS.md will help everyone using agents work
better with the codebase.

This AGENTS.md has thus far been working for me very successfully,
despite being simple. I suspect we'll add to it as time goes on but I
also want to avoid making it too large and polluting the context.

AGENTS.md

@@ -0,0 +1,23 @@
+# Agent Development Guide
+
+A file for [guiding coding agents](https://agents.md/).
+
+## Commands
+
+- **Build:** `zig build`
+- **Test (Zig):** `zig build test`
+- **Test filter (Zig)**: `zig build test -Dtest-filter=<test name>`
+- **Formatting (Zig)**: `zig fmt .`
+- **Formatting (other)**: `prettier -w .`
+
+## Directory Structure
+
+- Shared Zig core: `src/`
+- C API: `include/ghostty.h`
+- macOS app: `macos/`
+- GTK (Linux and FreeBSD) app: `src/apprt/gtk-ng`
+
+## macOS App
+
+- Do not use `xcodebuild`
+- Use `zig build` to build the macOS app and any shared Zig code