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/cmd_autotest.go

@@ -62,6 +62,8 @@ func runTest(portOverride string, baudOverride int, args []string) error {
 	if err != nil {
 		return fmt.Errorf("open %s: %w", port, err)
 	}
+	registerShutdown(func() { _ = sp.Close() })
+	enableShutdownOnInterrupt()
 	defer sp.Close()
 
 	if !*verbose {