Add config validation, remove command, status filtering, and unlock method display

config check: validates UUID format, recognized methods, keyfile
consistency and existence. Reports all issues with alias context.

remove: deletes a device from config by alias. Inverse of add.

status: --mounted, --unlocked, --locked flags filter the device table.
Flags combine as OR.

mount/unlock: display which method succeeded and key slot used, e.g.
"(fido2, key slot 1)". cryptsetup Open now runs with -v and parses
"Key slot N unlocked" from stderr via io.MultiWriter.

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

cmd/remove.go

@@ -0,0 +1,38 @@
+package cmd
+
+import (
+	"fmt"
+
+	"git.wntrmute.dev/kyle/arca/internal/config"
+	"github.com/spf13/cobra"
+)
+
+var removeCmd = &cobra.Command{
+	Use:               "remove <alias>",
+	Short:             "Remove a device from the config",
+	Args:              cobra.ExactArgs(1),
+	RunE:              runRemove,
+	ValidArgsFunction: completeDeviceOrAlias,
+}
+
+func init() {
+	rootCmd.AddCommand(removeCmd)
+}
+
+func runRemove(cmd *cobra.Command, args []string) error {
+	alias := args[0]
+	cfg := config.Load()
+
+	if _, ok := cfg.Devices[alias]; !ok {
+		return fmt.Errorf("no device with alias %q in config", alias)
+	}
+
+	delete(cfg.Devices, alias)
+
+	if err := cfg.Save(); err != nil {
+		return fmt.Errorf("saving config: %w", err)
+	}
+
+	fmt.Printf("Removed %q from config\n", alias)
+	return nil
+}