Close UART gracefully on SIGINT/SIGTERM in goTool.

Go exits on Ctrl+C without running defers, leaving the serial port locked; register shutdown hooks for serve and CLI commands.

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

goTool/main.go

@@ -62,6 +62,8 @@ func main() {
 		if err != nil {
 			log.Fatalf("open serial: %v", err)
 		}
+		registerShutdown(func() { _ = sp.Close() })
+		enableShutdownOnInterrupt()
 		defer sp.Close()
 		switch cmd {
 		case "version":