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/unlock.go

@@ -0,0 +1,59 @@
+package cmd
+
+import (
+	"fmt"
+
+	"git.wntrmute.dev/kyle/arca/internal/config"
+	"git.wntrmute.dev/kyle/arca/internal/udisks"
+	"git.wntrmute.dev/kyle/arca/internal/unlock"
+	"github.com/spf13/cobra"
+)
+
+var unlockCmd = &cobra.Command{
+	Use:               "unlock <device|alias>",
+	Short:             "Unlock a LUKS volume without mounting",
+	Args:              cobra.ExactArgs(1),
+	RunE:              runUnlock,
+	ValidArgsFunction: completeDeviceOrAlias,
+}
+
+func init() {
+	rootCmd.AddCommand(unlockCmd)
+}
+
+func runUnlock(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 unlocked.
+	if cleartext, err := client.CleartextDevice(dev); err == nil {
+		fmt.Printf("%s is already unlocked (%s)\n", target, cleartext.DevicePath)
+		return nil
+	}
+
+	u := unlock.New(client, unlock.Options{
+		ReadPassphrase: readPassphrase,
+		KeyfilePath:    devCfg.Keyfile,
+	})
+
+	result, err := u.Unlock(dev, devCfg.Methods)
+	if err != nil {
+		return err
+	}
+
+	fmt.Printf("Unlocked %s -> %s\n", target, result.Device.DevicePath)
+	return nil
+}