Add QEMU to dev environment and update testing workflow

- Add nix-env warning to AGENTS.md
- Include QEMU in flake.nix devShell for testing
- Update Makefile to use nix develop for QEMU commands
- Fix test-usb target to work with nix develop environment

AGENTS.md

@@ -58,3 +58,7 @@ This file provides guidelines for AI coding agents operating within this reposit
 ## ⚠️ Critical Warnings
 
 - **NEVER RUN `nix-env`** - This can break your Nix environment. Use `nix-shell`, `nix develop`, or declarative approaches instead.
+
+## ⚠️ Critical Warnings
+
+- **NEVER RUN `nix-env`** - This can break your Nix environment. Use `nix-shell`, `nix develop`, or declarative approaches instead.