feat(iso): add nogpu variant (no NVIDIA, no AMD/ROCm)

- build.sh: accept --variant nogpu; skips all GPU build steps, removes
  both nvidia-cuda and rocm archives, strips bee-nvidia-load and
  bee-nvidia.service from overlay
- build-in-container.sh: add nogpu to --variant flag; all variant
  includes nogpu; --clean-build wipes live-build-work-nogpu
- 9000-bee-setup hook: nogpu path enables no GPU services
- bee-nogpu.list.chroot: empty GPU package list

Output: easy-bee-nogpu-vX.iso

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

iso/builder/build-in-container.sh

@@ -48,8 +48,8 @@ while [ $# -gt 0 ]; do
 done
 
 case "$VARIANT" in
-    nvidia|amd|all) ;;
-    *) echo "unknown variant: $VARIANT (expected nvidia, amd, or all)" >&2; exit 1 ;;
+    nvidia|amd|nogpu|all) ;;
+    *) echo "unknown variant: $VARIANT (expected nvidia, amd, nogpu, or all)" >&2; exit 1 ;;
 esac
 
 if [ "$CLEAN_CACHE" = "1" ]; then
@@ -62,6 +62,7 @@ if [ "$CLEAN_CACHE" = "1" ]; then
     echo "=== cleaning live-build work dirs ==="
     rm -rf "${REPO_ROOT}/dist/live-build-work-nvidia"
     rm -rf "${REPO_ROOT}/dist/live-build-work-amd"
+    rm -rf "${REPO_ROOT}/dist/live-build-work-nogpu"
     echo "=== caches cleared, proceeding with build ==="
 fi
 
@@ -182,8 +183,12 @@ case "$VARIANT" in
     amd)
         run_variant amd
         ;;
+    nogpu)
+        run_variant nogpu
+        ;;
     all)
         run_variant nvidia
         run_variant amd
+        run_variant nogpu
         ;;
 esac