Persist undo/redo history across runs with a 20-entry cap.

Serialize create/update/delete commands to DataDir/history.json and reload them on startup via OpenHistory.

Co-authored-by: Cursor <cursoragent@cursor.com>

cmd/task-master/main.go

@@ -48,7 +48,10 @@ func run() error {
 		return fmt.Errorf("ensure project: %w", err)
 	}
 
-	history := command.NewHistory()
+	history, err := command.OpenHistory(command.HistoryPath(cfg.DataDir), store)
+	if err != nil {
+		return fmt.Errorf("history: %w", err)
+	}
 	model := ui.New(cfg, store, projects, history)
 	prog := tea.NewProgram(model, tea.WithAltScreen())
 	if _, err := prog.Run(); err != nil {