feat: add cloud-init Go parser (Phase 2)

Implement a lightweight cloud-init system for first-boot configuration:
- Go parser for YAML config (hostname, network, KubeSolo settings)
- Static/DHCP network modes with DNS override
- KubeSolo extra flags and API server SAN configuration
- Portainer Edge Agent and air-gapped deployment support
- New init stage 45-cloud-init.sh runs before network/hostname stages
- Stages 50/60 skip gracefully when cloud-init has already applied
- Build script compiles static Linux/amd64 binary (~2.7 MB)
- 17 unit tests covering parsing, validation, and example files
- Full documentation at docs/cloud-init.md

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

build/scripts/inject-kubesolo.sh

@@ -63,6 +63,16 @@ for lib in network.sh health.sh; do
     [ -f "$src" ] && cp "$src" "$ROOTFS/usr/lib/kubesolo-os/$lib"
 done
 
+# Cloud-init binary (Go, built separately)
+CLOUDINIT_BIN="$CACHE_DIR/kubesolo-cloudinit"
+if [ -f "$CLOUDINIT_BIN" ]; then
+    cp "$CLOUDINIT_BIN" "$ROOTFS/usr/lib/kubesolo-os/kubesolo-cloudinit"
+    chmod +x "$ROOTFS/usr/lib/kubesolo-os/kubesolo-cloudinit"
+    echo "    Installed cloud-init binary ($(du -h "$CLOUDINIT_BIN" | cut -f1))"
+else
+    echo "    WARN: Cloud-init binary not found (run 'make build-cloudinit' to build)"
+fi
+
 # --- 3. Kernel modules list ---
 cp "$PROJECT_ROOT/build/config/modules.list" "$ROOTFS/usr/lib/kubesolo-os/modules.list"