Add dual-path ISO build scripts (Arch + Debian) with README

- iso/build.sh: Debian/Ubuntu host builder using debootstrap + xorriso
  Bootstraps Debian bookworm, compiles OrionOS in chroot, produces
  bootable hybrid ISO with GRUB (BIOS + EFI), auto-login root on tty1
- iso/build-arch.sh: Arch Linux host builder using mkarchiso
  Compiles OrionOS, installs into archiso profile, runs mkarchiso
- iso/README.md: instructions for all three paths (Docker/Debian/Arch)
  including dd and Ventoy USB flashing
- Makefile: iso target auto-detects available tool (docker/debootstrap/mkarchiso)

Makefile

@@ -102,7 +102,18 @@ install: all
 	@echo "  exec orionwm"
 
 iso:
-	@echo "Building OrionOS ISO via Docker..."
-	docker build -t orionos-iso-builder -f iso/docker/Dockerfile .
-	mkdir -p iso/output
-	docker run --rm --privileged -v "$(PWD)/iso/output:/output" orionos-iso-builder
+	@if command -v docker >/dev/null 2>&1 && docker info >/dev/null 2>&1; then \
+	    echo "Building OrionOS ISO via Docker (Arch)..."; \
+	    docker build -t orionos-iso-builder -f iso/docker/Dockerfile .; \
+	    mkdir -p iso/output; \
+	    docker run --rm --privileged -v "$$(pwd)/iso/output:/output" orionos-iso-builder; \
+	elif command -v debootstrap >/dev/null 2>&1; then \
+	    echo "Building OrionOS ISO via debootstrap (Debian/Ubuntu)..."; \
+	    sudo bash iso/build.sh; \
+	elif command -v mkarchiso >/dev/null 2>&1; then \
+	    echo "Building OrionOS ISO via mkarchiso (Arch native)..."; \
+	    sudo bash iso/build-arch.sh; \
+	else \
+	    echo "No ISO build tool found. See iso/README.md for options."; \
+	    exit 1; \
+	fi