feat(cli): add promote, cards, copy, demote, delete, edit commands

Complete CLI crystallization loop. Promote generates card_data
(template slots, checklist steps, link URLs). Cards view sorted by
use_count. Copy increments usage. Demote strips card layer. Delete
does soft then hard. Edit opens $EDITOR.

cmd/demote.go

@@ -0,0 +1,43 @@
+package cmd
+
+import (
+	"fmt"
+
+	"github.com/lerko/nib/internal/db"
+	"github.com/lerko/nib/internal/display"
+	"github.com/spf13/cobra"
+)
+
+var demoteCmd = &cobra.Command{
+	Use:   "demote <id>",
+	Short: "strip card layer, return to fluid",
+	Args:  cobra.ExactArgs(1),
+	RunE:  runDemote,
+}
+
+func init() {
+	rootCmd.AddCommand(demoteCmd)
+}
+
+func runDemote(_ *cobra.Command, args []string) error {
+	store, err := openStore()
+	if err != nil {
+		return err
+	}
+	defer store.Close()
+
+	id, err := store.Resolve(args[0])
+	if err != nil {
+		return fmt.Errorf("not_found — no entity with id %s", args[0])
+	}
+
+	if err := store.Demote(id); err != nil {
+		if err == db.ErrAlreadyFluid {
+			return fmt.Errorf("invalid_demote — entity %s is already fluid", display.FormatID(id))
+		}
+		return err
+	}
+
+	fmt.Printf("demoted %s → note\n", display.FormatID(id))
+	return nil
+}