Add unlock and lock commands for decrypt-only operations

unlock: decrypts a LUKS volume without mounting. Idempotent — reports
existing cleartext device if already unlocked.

lock: locks a LUKS volume. If mounted, unmounts first (udisks2 with
privileged fallback) then locks. Idempotent — reports if already locked.

Both commands support alias completion and config resolution.

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

cmd/lock.go

@@ -0,0 +1,68 @@
+package cmd
+
+import (
+	"fmt"
+
+	"git.wntrmute.dev/kyle/arca/internal/config"
+	"git.wntrmute.dev/kyle/arca/internal/cryptsetup"
+	"git.wntrmute.dev/kyle/arca/internal/udisks"
+	"github.com/spf13/cobra"
+)
+
+var lockCmd = &cobra.Command{
+	Use:               "lock <device|alias>",
+	Short:             "Lock a LUKS volume without unmounting",
+	Long:              "Locks (closes) a LUKS volume. If the volume is mounted, it will be unmounted first.",
+	Args:              cobra.ExactArgs(1),
+	RunE:              runLock,
+	ValidArgsFunction: completeDeviceOrAlias,
+}
+
+func init() {
+	rootCmd.AddCommand(lockCmd)
+}
+
+func runLock(cmd *cobra.Command, args []string) error {
+	cfg := config.Load()
+	target := args[0]
+
+	client, err := udisks.NewClient()
+	if err != nil {
+		return fmt.Errorf("connecting to udisks2: %w", err)
+	}
+	defer client.Close()
+
+	devCfg := cfg.ResolveDevice(target)
+
+	dev, err := client.FindDevice(devCfg.UUID, target)
+	if err != nil {
+		return err
+	}
+
+	// Check if already locked.
+	cleartext, err := client.CleartextDevice(dev)
+	if err != nil {
+		fmt.Fprintf(cmd.ErrOrStderr(), "%s is already locked\n", target)
+		return nil
+	}
+
+	// If mounted, unmount first — can't lock a mounted device.
+	if mp, mounted := client.IsMounted(cleartext); mounted {
+		if err := client.Unmount(cleartext); err != nil {
+			if err := cryptsetup.Unmount(mp); err != nil {
+				return fmt.Errorf("unmounting before lock: %w", err)
+			}
+		}
+	}
+
+	// Lock: try udisks2 first, fall back to cryptsetup close.
+	if err := client.Lock(dev); err != nil {
+		mapperName := cryptsetup.MapperName(dev.DevicePath)
+		if err := cryptsetup.Close(mapperName); err != nil {
+			return fmt.Errorf("locking: %w", err)
+		}
+	}
+
+	fmt.Printf("Locked %s\n", target)
+	return nil
+}