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/45-cloud-init.sh

@@ -0,0 +1,35 @@
+#!/bin/sh
+# 45-cloud-init.sh — Apply cloud-init configuration
+#
+# Runs the kubesolo-cloudinit binary to parse cloud-init.yaml and apply:
+#   - hostname (/etc/hostname, /etc/hosts)
+#   - network (static IP or DHCP)
+#   - KubeSolo settings (/etc/kubesolo/extra-flags, config.yaml)
+#   - persistent configs saved to data partition
+#
+# If no cloud-init file is found, this stage is a no-op and later stages
+# (50-network, 60-hostname) handle defaults.
+
+CLOUDINIT_BIN="/usr/lib/kubesolo-os/kubesolo-cloudinit"
+CLOUDINIT_FILE="${KUBESOLO_CLOUDINIT:-$DATA_MOUNT/etc-kubesolo/cloud-init.yaml}"
+
+if [ ! -x "$CLOUDINIT_BIN" ]; then
+    log_warn "cloud-init binary not found at $CLOUDINIT_BIN — skipping"
+    return 0
+fi
+
+if [ ! -f "$CLOUDINIT_FILE" ]; then
+    log "No cloud-init config found at $CLOUDINIT_FILE — skipping"
+    return 0
+fi
+
+log "Applying cloud-init from: $CLOUDINIT_FILE"
+
+if "$CLOUDINIT_BIN" apply "$CLOUDINIT_FILE"; then
+    log_ok "cloud-init applied successfully"
+    # Signal to later stages that cloud-init handled network/hostname
+    CLOUDINIT_APPLIED=1
+    export CLOUDINIT_APPLIED
+else
+    log_err "cloud-init apply failed — later stages will use defaults"
+fi