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/config/hooks/normal/9000-bee-setup.hook.chroot

@@ -50,6 +50,7 @@ elif [ "$GPU_VENDOR" = "amd" ]; then
         fi
     done
 fi
+# nogpu: no GPU services needed
 
 # Ensure scripts are executable
 chmod +x /usr/local/bin/bee-network.sh  2>/dev/null || true