M9: init --merge to add new devices without overwriting

Add --merge flag to init that loads existing config, skips devices
whose UUID is already configured, and appends only new discoveries.
--force and --merge are mutually exclusive. Uses Config.Save() from M8.
Error message now suggests both --force and --merge.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

cmd/init.go

@@ -3,16 +3,17 @@ package cmd
 import (
 	"fmt"
 	"os"
-	"path/filepath"
 
 	"git.wntrmute.dev/kyle/arca/internal/config"
 	"git.wntrmute.dev/kyle/arca/internal/udisks"
 	"github.com/godbus/dbus/v5"
 	"github.com/spf13/cobra"
-	"gopkg.in/yaml.v3"
 )
 
-var forceInit bool
+var (
+	forceInit bool
+	mergeInit bool
+)
 
 var initCmd = &cobra.Command{
 	Use:   "init",
@@ -23,17 +24,26 @@ var initCmd = &cobra.Command{
 
 func init() {
 	initCmd.Flags().BoolVarP(&forceInit, "force", "f", false, "overwrite existing config file")
+	initCmd.Flags().BoolVar(&mergeInit, "merge", false, "add new devices to existing config without overwriting")
+	initCmd.MarkFlagsMutuallyExclusive("force", "merge")
 	rootCmd.AddCommand(initCmd)
 }
 
 func runInit(cmd *cobra.Command, args []string) error {
 	cfgPath := config.Path()
 
+	// Load existing config for merge, or start fresh.
+	var cfg *config.Config
+	if mergeInit {
+		cfg = config.Load()
+	} else {
 		if !forceInit {
 			if _, err := os.Stat(cfgPath); err == nil {
-			return fmt.Errorf("config already exists at %s (use --force to overwrite)", cfgPath)
+				return fmt.Errorf("config already exists at %s (use --force to overwrite or --merge to add new devices)", cfgPath)
 			}
 		}
+		cfg = &config.Config{Devices: make(map[string]config.DeviceConfig)}
+	}
 
 	client, err := udisks.NewClient()
 	if err != nil {
@@ -51,43 +61,42 @@ func runInit(cmd *cobra.Command, args []string) error {
 		return fmt.Errorf("detecting root device: %w", err)
 	}
 
-	cfg := config.Config{
+	added := 0
-		Devices: make(map[string]config.DeviceConfig),
-	}
-
 	for _, dev := range encrypted {
 		if isRootBacking(dev.ObjectPath, rootBacking) {
 			fmt.Fprintf(os.Stderr, "Skipping %s (root filesystem)\n", dev.DevicePath)
 			continue
 		}
 
+		if cfg.HasUUID(dev.UUID) {
+			fmt.Fprintf(os.Stderr, "Skipping %s (already configured)\n", dev.DevicePath)
+			continue
+		}
+
 		alias := aliasFromUUID(dev.UUID)
 		cfg.Devices[alias] = config.DeviceConfig{
 			UUID:    dev.UUID,
 			Methods: []string{"passphrase"},
 		}
 		fmt.Fprintf(os.Stderr, "Found %s (UUID %s) -> alias %q\n", dev.DevicePath, dev.UUID, alias)
+		added++
 	}
 
-	if len(cfg.Devices) == 0 {
+	if added == 0 && !mergeInit {
 		fmt.Println("No non-root LUKS devices found.")
 		return nil
 	}
 
-	data, err := yaml.Marshal(&cfg)
+	if added == 0 && mergeInit {
-	if err != nil {
+		fmt.Println("No new devices to add.")
-		return fmt.Errorf("marshaling config: %w", err)
+		return nil
 	}
 
-	if err := os.MkdirAll(filepath.Dir(cfgPath), 0o755); err != nil {
+	if err := cfg.Save(); err != nil {
-		return fmt.Errorf("creating config directory: %w", err)
+		return fmt.Errorf("saving config: %w", err)
 	}
 
-	if err := os.WriteFile(cfgPath, data, 0o644); err != nil {
+	fmt.Printf("Config written to %s (%d device(s) added)\n", cfgPath, added)
-		return fmt.Errorf("writing config: %w", err)
-	}
-
-	fmt.Printf("Config written to %s\n", cfgPath)
 	return nil
 }
 
@@ -99,4 +108,3 @@ func isRootBacking(path dbus.ObjectPath, rootDevices []dbus.ObjectPath) bool {
 	}
 	return false
 }
-