perf: skip go rebuild if sources unchanged, use rsync for ISO download

- audit binary is only rebuilt when .go files are newer than the binary
- rsync replaces scp for ISO download (delta transfer on repeat builds)

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

scripts/run-builder.sh

@@ -56,7 +56,8 @@ echo ""
 echo "=== downloading ISO ==="
 LOCAL_ISO_DIR="${REPO_ROOT}/iso/out"
 mkdir -p "${LOCAL_ISO_DIR}"
-scp -o StrictHostKeyChecking=no \
+rsync -az --progress \
+    -e "ssh -o StrictHostKeyChecking=no" \
     "root@${BUILDER_HOST}:/root/bee/dist/*.iso" \
     "${LOCAL_ISO_DIR}/"
 echo ""