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/parser.go

@@ -0,0 +1,54 @@
+package cloudinit
+
+import (
+	"fmt"
+	"os"
+
+	"gopkg.in/yaml.v3"
+)
+
+// Parse reads a cloud-init YAML file and returns the parsed config.
+func Parse(path string) (*Config, error) {
+	data, err := os.ReadFile(path)
+	if err != nil {
+		return nil, fmt.Errorf("reading cloud-init file %s: %w", path, err)
+	}
+	return ParseBytes(data)
+}
+
+// ParseBytes parses cloud-init YAML from a byte slice.
+func ParseBytes(data []byte) (*Config, error) {
+	var cfg Config
+	if err := yaml.Unmarshal(data, &cfg); err != nil {
+		return nil, fmt.Errorf("parsing cloud-init YAML: %w", err)
+	}
+
+	if err := validate(&cfg); err != nil {
+		return nil, fmt.Errorf("validating cloud-init config: %w", err)
+	}
+
+	// Apply defaults
+	if cfg.Network.Mode == "" {
+		cfg.Network.Mode = "dhcp"
+	}
+
+	return &cfg, nil
+}
+
+func validate(cfg *Config) error {
+	switch cfg.Network.Mode {
+	case "", "dhcp":
+		// valid
+	case "static":
+		if cfg.Network.Address == "" {
+			return fmt.Errorf("static network mode requires 'address' field")
+		}
+		if cfg.Network.Gateway == "" {
+			return fmt.Errorf("static network mode requires 'gateway' field")
+		}
+	default:
+		return fmt.Errorf("unknown network mode: %q (expected 'dhcp' or 'static')", cfg.Network.Mode)
+	}
+
+	return nil
+}