feat: add disk-deploy core, disk image builder, and installer

- deploy-disk.sh: the shared "write a finished system to a disk" step —
  partition (GPT bios/efi) -> btrfs layout via btrfs/init.sh -> rsync
  rootfs -> chroot (real fstab, GRUB, hostname). Carries the optional
  ARCLINE_SIGN hook; the signing tooling itself lands in a later commit.
- apply-fstab.sh: renders the edition fstab template with real root/efi
  UUIDs and drops the swap line.
- build-image.sh: rootfs -> bootable qcow2/raw disk image (sparse file +
  loop device + deploy), the cloud edition's primary output.
- install.sh: scripted installer for a real disk, confirmation-gated.
- Makefile: image-<edition> targets (+ minimal variants); build-edition.sh
  learns the "image" stage; cloud metadata now ships a disk image.
- check-host-deps.sh / GitLab CI: add gdisk, parted, rsync, dosfstools,
  qemu-utils, dpkg, sbsigntool to the build image.

scripts/build-edition.sh

@@ -21,7 +21,10 @@ case "$STAGE" in
   iso)
     "$ROOT/scripts/build-iso.sh" "$EDITION"
     ;;
+  image)
+    "$ROOT/scripts/build-image.sh" "$EDITION"
+    ;;
   *)
-    die "unknown stage '$STAGE' (expected: rootfs | iso)"
+    die "unknown stage '$STAGE' (expected: rootfs | iso | image)"
     ;;
 esac