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
+}