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/edit.go

@@ -0,0 +1,83 @@
+package cmd
+
+import (
+	"fmt"
+	"os"
+	"os/exec"
+
+	"github.com/lerko/nib/internal/db"
+	"github.com/lerko/nib/internal/display"
+	"github.com/spf13/cobra"
+)
+
+var editCmd = &cobra.Command{
+	Use:   "edit <id>",
+	Short: "edit entity body in $EDITOR",
+	Args:  cobra.ExactArgs(1),
+	RunE:  runEdit,
+}
+
+func init() {
+	rootCmd.AddCommand(editCmd)
+}
+
+func runEdit(_ *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])
+	}
+
+	e, err := store.Get(id)
+	if err != nil {
+		return err
+	}
+
+	tmpfile, err := os.CreateTemp("", "nib-*.md")
+	if err != nil {
+		return err
+	}
+	defer os.Remove(tmpfile.Name())
+
+	if _, err := tmpfile.WriteString(e.Body); err != nil {
+		tmpfile.Close()
+		return err
+	}
+	tmpfile.Close()
+
+	editor := os.Getenv("EDITOR")
+	if editor == "" {
+		editor = "vi"
+	}
+
+	cmd := exec.Command(editor, tmpfile.Name())
+	cmd.Stdin = os.Stdin
+	cmd.Stdout = os.Stdout
+	cmd.Stderr = os.Stderr
+	if err := cmd.Run(); err != nil {
+		return fmt.Errorf("editor: %w", err)
+	}
+
+	newBody, err := os.ReadFile(tmpfile.Name())
+	if err != nil {
+		return err
+	}
+
+	body := string(newBody)
+	if body == e.Body {
+		fmt.Println("(no changes)")
+		return nil
+	}
+
+	if err := store.Update(id, &db.EntityUpdate{Body: &body}); err != nil {
+		return err
+	}
+
+	fmt.Printf("updated %s\n", display.FormatID(id))
+	return nil
+}