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.

Makefile

@@ -59,9 +59,9 @@ flash-usb: build-usb
 	sync
 	@echo "✅ USB drive ready!"
 
-test-usb: 
+test-usb:
 	@echo "🧪 Testing USB environment in QEMU..."
-	qemu-system-x86_64 \
+	nix develop --command qemu-system-x86_64 \
 		-cdrom $(ISO_FILE) \
 		-m 2048 \
 		-enable-kvm \

flake.nix

@@ -65,6 +65,7 @@
           markdownlint-cli
           jq
           nixpkgs-fmt
+          qemu
         ];
       };