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>

cloud-init/network.go

@@ -0,0 +1,174 @@
+package cloudinit
+
+import (
+	"fmt"
+	"log/slog"
+	"os"
+	"os/exec"
+	"path/filepath"
+	"strings"
+)
+
+// ApplyNetwork configures the network interface based on cloud-init config.
+// For static mode, it sets the IP, gateway, and DNS directly.
+// For DHCP mode, it runs udhcpc on the target interface.
+func ApplyNetwork(cfg *Config) error {
+	iface := cfg.Network.Interface
+	if iface == "" {
+		var err error
+		iface, err = detectPrimaryInterface()
+		if err != nil {
+			return fmt.Errorf("detecting primary interface: %w", err)
+		}
+	}
+
+	slog.Info("configuring network", "interface", iface, "mode", cfg.Network.Mode)
+
+	// Bring up the interface
+	if err := run("ip", "link", "set", iface, "up"); err != nil {
+		return fmt.Errorf("bringing up %s: %w", iface, err)
+	}
+
+	switch cfg.Network.Mode {
+	case "static":
+		return applyStatic(iface, cfg)
+	case "dhcp", "":
+		return applyDHCP(iface, cfg)
+	default:
+		return fmt.Errorf("unknown network mode: %s", cfg.Network.Mode)
+	}
+}
+
+func applyStatic(iface string, cfg *Config) error {
+	// Set IP address
+	if err := run("ip", "addr", "add", cfg.Network.Address, "dev", iface); err != nil {
+		return fmt.Errorf("setting address %s on %s: %w", cfg.Network.Address, iface, err)
+	}
+
+	// Set default gateway
+	if err := run("ip", "route", "add", "default", "via", cfg.Network.Gateway, "dev", iface); err != nil {
+		return fmt.Errorf("setting gateway %s: %w", cfg.Network.Gateway, err)
+	}
+
+	// Write DNS configuration
+	if len(cfg.Network.DNS) > 0 {
+		if err := writeDNS(cfg.Network.DNS); err != nil {
+			return fmt.Errorf("writing DNS config: %w", err)
+		}
+	}
+
+	slog.Info("static network configured",
+		"interface", iface,
+		"address", cfg.Network.Address,
+		"gateway", cfg.Network.Gateway,
+	)
+	return nil
+}
+
+func applyDHCP(iface string, cfg *Config) error {
+	// Try udhcpc (BusyBox), then dhcpcd
+	if path, err := exec.LookPath("udhcpc"); err == nil {
+		args := []string{"-i", iface, "-s", "/usr/share/udhcpc/default.script",
+			"-t", "10", "-T", "3", "-A", "5", "-b", "-q"}
+		if err := run(path, args...); err != nil {
+			return fmt.Errorf("udhcpc on %s: %w", iface, err)
+		}
+	} else if path, err := exec.LookPath("dhcpcd"); err == nil {
+		if err := run(path, iface); err != nil {
+			return fmt.Errorf("dhcpcd on %s: %w", iface, err)
+		}
+	} else {
+		return fmt.Errorf("no DHCP client available (need udhcpc or dhcpcd)")
+	}
+
+	// Override DNS if specified in config
+	if len(cfg.Network.DNS) > 0 {
+		if err := writeDNS(cfg.Network.DNS); err != nil {
+			return fmt.Errorf("writing DNS config: %w", err)
+		}
+	}
+
+	slog.Info("DHCP network configured", "interface", iface)
+	return nil
+}
+
+// SaveNetworkConfig writes a shell script that restores the current network
+// config on subsequent boots (before cloud-init runs).
+func SaveNetworkConfig(cfg *Config, destDir string) error {
+	iface := cfg.Network.Interface
+	if iface == "" {
+		var err error
+		iface, err = detectPrimaryInterface()
+		if err != nil {
+			return err
+		}
+	}
+
+	if err := os.MkdirAll(destDir, 0o755); err != nil {
+		return fmt.Errorf("creating network config dir: %w", err)
+	}
+
+	dest := filepath.Join(destDir, "interfaces.sh")
+	var sb strings.Builder
+	sb.WriteString("#!/bin/sh\n")
+	sb.WriteString("# Auto-generated by KubeSolo OS cloud-init\n")
+	sb.WriteString(fmt.Sprintf("ip link set %s up\n", iface))
+
+	switch cfg.Network.Mode {
+	case "static":
+		sb.WriteString(fmt.Sprintf("ip addr add %s dev %s\n", cfg.Network.Address, iface))
+		sb.WriteString(fmt.Sprintf("ip route add default via %s dev %s\n", cfg.Network.Gateway, iface))
+		if len(cfg.Network.DNS) > 0 {
+			sb.WriteString(": > /etc/resolv.conf\n")
+			for _, ns := range cfg.Network.DNS {
+				sb.WriteString(fmt.Sprintf("echo 'nameserver %s' >> /etc/resolv.conf\n", ns))
+			}
+		}
+	case "dhcp", "":
+		sb.WriteString("udhcpc -i " + iface + " -s /usr/share/udhcpc/default.script -t 10 -T 3 -A 5 -b -q 2>/dev/null\n")
+	}
+
+	if err := os.WriteFile(dest, []byte(sb.String()), 0o755); err != nil {
+		return fmt.Errorf("writing network config: %w", err)
+	}
+
+	slog.Info("network config saved", "path", dest)
+	return nil
+}
+
+func writeDNS(servers []string) error {
+	var sb strings.Builder
+	for _, ns := range servers {
+		sb.WriteString("nameserver " + ns + "\n")
+	}
+	return os.WriteFile("/etc/resolv.conf", []byte(sb.String()), 0o644)
+}
+
+func detectPrimaryInterface() (string, error) {
+	entries, err := os.ReadDir("/sys/class/net")
+	if err != nil {
+		return "", fmt.Errorf("reading /sys/class/net: %w", err)
+	}
+	for _, e := range entries {
+		name := e.Name()
+		switch {
+		case name == "lo",
+			strings.HasPrefix(name, "docker"),
+			strings.HasPrefix(name, "veth"),
+			strings.HasPrefix(name, "br"),
+			strings.HasPrefix(name, "cni"),
+			strings.HasPrefix(name, "flannel"),
+			strings.HasPrefix(name, "cali"):
+			continue
+		}
+		return name, nil
+	}
+	return "", fmt.Errorf("no suitable network interface found")
+}
+
+func run(name string, args ...string) error {
+	cmd := exec.Command(name, args...)
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	return cmd.Run()
+}