M8: add command to append a single device to config

New 'arca add <device>' subcommand detects a LUKS device via udisks2 and
appends it to the config with passphrase as default method. Supports
--alias/-a to override the generated name. Skips if UUID already
configured. Adds Config.Save() and Config.HasUUID() to config package.

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

cmd/add.go

@@ -0,0 +1,87 @@
+package cmd
+
+import (
+	"fmt"
+	"strings"
+
+	"git.wntrmute.dev/kyle/arca/internal/config"
+	"git.wntrmute.dev/kyle/arca/internal/udisks"
+	"github.com/spf13/cobra"
+)
+
+var addAlias string
+
+var addCmd = &cobra.Command{
+	Use:   "add <device>",
+	Short: "Add a device to the config",
+	Long:  "Detects a LUKS device via udisks2 and adds it to the config file with a default passphrase method.",
+	Args:  cobra.ExactArgs(1),
+	RunE:  runAdd,
+}
+
+func init() {
+	addCmd.Flags().StringVarP(&addAlias, "alias", "a", "", "alias name (default: first 8 chars of UUID)")
+	rootCmd.AddCommand(addCmd)
+}
+
+func runAdd(cmd *cobra.Command, args []string) error {
+	target := args[0]
+
+	client, err := udisks.NewClient()
+	if err != nil {
+		return fmt.Errorf("connecting to udisks2: %w", err)
+	}
+	defer client.Close()
+
+	// Find the device to get its UUID.
+	dev, err := client.FindDevice("", target)
+	if err != nil {
+		return err
+	}
+
+	if !dev.HasEncrypted {
+		return fmt.Errorf("%s is not a LUKS-encrypted device", target)
+	}
+
+	if dev.UUID == "" {
+		return fmt.Errorf("%s has no UUID", target)
+	}
+
+	cfg := config.Load()
+
+	// Check if already configured.
+	if existing := cfg.AliasFor(dev.UUID); existing != "" {
+		fmt.Printf("Device %s (UUID %s) already configured as %q\n", dev.DevicePath, dev.UUID, existing)
+		return nil
+	}
+
+	alias := addAlias
+	if alias == "" {
+		alias = aliasFromUUID(dev.UUID)
+	}
+
+	// Check for alias collision.
+	if _, exists := cfg.Devices[alias]; exists {
+		return fmt.Errorf("alias %q already in use — choose a different name with --alias", alias)
+	}
+
+	cfg.Devices[alias] = config.DeviceConfig{
+		UUID:    dev.UUID,
+		Methods: []string{"passphrase"},
+	}
+
+	if err := cfg.Save(); err != nil {
+		return fmt.Errorf("saving config: %w", err)
+	}
+
+	fmt.Printf("Added %s (UUID %s) as %q\n", dev.DevicePath, dev.UUID, alias)
+	return nil
+}
+
+func aliasFromUUID(uuid string) string {
+	clean := strings.ReplaceAll(uuid, "-", "")
+	if len(clean) > 8 {
+		clean = clean[:8]
+	}
+	return clean
+}

cmd/init.go

@@ -4,8 +4,6 @@ import (
 	"fmt"
 	"os"
 	"path/filepath"
-	"strings"
-
 
 	"git.wntrmute.dev/kyle/arca/internal/config"
 	"git.wntrmute.dev/kyle/arca/internal/udisks"
@@ -102,12 +100,3 @@ func isRootBacking(path dbus.ObjectPath, rootDevices []dbus.ObjectPath) bool {
 	return false
 }
 
-func aliasFromUUID(uuid string) string {
-	// Use first 8 chars of UUID as a stable alias.
-	// "b8b2f8e3-4cde-4aca-a96e-df9274019f9f" -> "b8b2f8e3"
-	clean := strings.ReplaceAll(uuid, "-", "")
-	if len(clean) > 8 {
-		clean = clean[:8]
-	}
-	return clean
-}

internal/config/config.go

@@ -1,6 +1,7 @@
 package config
 
 import (
+	"fmt"
 	"os"
 	"path/filepath"
 
@@ -76,6 +77,31 @@ func resolvedFrom(dev DeviceConfig) ResolvedDevice {
 	}
 }
 
+// HasUUID returns true if a device with the given UUID is already configured.
+func (c *Config) HasUUID(uuid string) bool {
+	for _, dev := range c.Devices {
+		if dev.UUID == uuid {
+			return true
+		}
+	}
+	return false
+}
+
+// Save writes the config to the config file.
+func (c *Config) Save() error {
+	path := configPath()
+	if err := os.MkdirAll(filepath.Dir(path), 0o755); err != nil {
+		return fmt.Errorf("creating config directory: %w", err)
+	}
+
+	data, err := yaml.Marshal(c)
+	if err != nil {
+		return fmt.Errorf("marshaling config: %w", err)
+	}
+
+	return os.WriteFile(path, data, 0o644)
+}
+
 // AliasFor returns the config alias for a given UUID, or "" if none.
 func (c *Config) AliasFor(uuid string) string {
 	for name, dev := range c.Devices {