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>

init/lib/60-hostname.sh

@@ -1,5 +1,15 @@
 #!/bin/sh
 # 60-hostname.sh — Set system hostname
+# If cloud-init (stage 45) already set the hostname, skip this stage.
+
+# Cloud-init writes /etc/hostname and saves to data partition
+if [ "$CLOUDINIT_APPLIED" = "1" ] && [ -f /etc/hostname ]; then
+    HOSTNAME="$(cat /etc/hostname)"
+    if [ -n "$HOSTNAME" ]; then
+        log "Hostname already set by cloud-init: $HOSTNAME"
+        return 0
+    fi
+fi
 
 if [ -f "$DATA_MOUNT/etc-kubesolo/hostname" ]; then
     HOSTNAME="$(cat "$DATA_MOUNT/etc-kubesolo/hostname")"