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)

iso/README.md

@@ -0,0 +1,51 @@
+# OrionOS ISO Builder
+
+Two build paths are provided. Pick the one that matches your host.
+
+## Path A — Arch Linux host (native, recommended)
+
+Requires `archiso` and `pacman-contrib`.
+
+```bash
+sudo pacman -S archiso
+sudo make iso          # runs iso/build-arch.sh
+```
+
+## Path B — Debian / Ubuntu host
+
+Requires `xorriso`, `squashfs-tools`, `debootstrap`, `grub-pc-bin`, `grub-efi-amd64-bin`.
+
+```bash
+sudo apt-get install -y xorriso squashfs-tools debootstrap \
+    grub-pc-bin grub-efi-amd64-bin mtools
+sudo bash iso/build.sh
+```
+
+Output lands in `iso/output/OrionOS-YYYYMMDD.iso`.
+
+## Path C — Docker (any host with Docker)
+
+```bash
+make iso
+```
+
+The Dockerfile (`iso/docker/Dockerfile`) builds inside `archlinux:latest`,
+compiles OrionOS, and runs `mkarchiso`. Output is written to `iso/output/`.
+
+## Writing the ISO to a USB drive
+
+```bash
+# Find your USB device (e.g. /dev/sdb) — be careful
+lsblk
+sudo dd if=iso/output/OrionOS-*.iso of=/dev/sdX bs=4M status=progress oflag=sync
+```
+
+Or use [Ventoy](https://ventoy.net) — just copy the `.iso` onto the Ventoy USB.
+
+## First boot
+
+- Logs in automatically as root on tty1 and starts X / OrionOS.
+- Super+F1 or bottom-left hot corner → launcher
+- Super+V → clipboard manager
+- Print → screenshot (Shift+Print → region)
+- Top-left hot corner → Exposé overview