refactor(ci): customize cloud image on nested PVE instead of container

Move virt-customize to run on the nested PVE host (which already has
libguestfs) instead of inside the CI container. This removes
libguestfs-tools, linux-image-generic, and qemu-utils from the
Dockerfile, shrinking the image from ~1.2GB to ~770MB.

- prepare-test-vm.sh: downloads Debian cloud image on nested PVE,
  runs virt-customize there to install qemu-guest-agent, then creates
  VM with imported disk + cloud-init
- Remove build-test-image.sh (no longer needed)
- Remove /opt/pve-images volume mount

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

tests/infrastructure/Dockerfile

@@ -15,9 +15,6 @@ RUN apt-get update && apt-get install -y --no-install-recommends \
     lsb-release \
     ca-certificates \
     python3 \
-    libguestfs-tools \
-    linux-image-generic \
-    qemu-utils \
     xorriso \
     && rm -rf /var/lib/apt/lists/*
 
@@ -64,7 +61,5 @@ RUN echo "=== Tool verification ===" && \
     curl --version | head -1 && \
     jq --version && \
     python3 --version && \
-    virt-customize --version && \
     ssh -V 2>&1 && \
-    scp 2>&1 | head -1 || true && \
     echo "=== All tools verified ==="