fix: use UTF-8 BOM for .bat so cmd shows Cyrillic correctly

UTF-8 without BOM + chcp 65001 showed '????' because cmd reads .bat under the
OEM codepage, not chcp. A UTF-8 BOM makes cmd read the file as UTF-8 regardless
of the console codepage; verified Cyrillic in echo/set/for displays correctly
even when cmd starts at cp866. AGENTS.md updated accordingly.

scripts/open-firewall.bat

@@ -1,4 +1,4 @@
-@echo off
+@echo off
 REM ============================================================
 REM  open-firewall.bat - открывает порт 8080 в Windows Firewall.
 REM  Запускать от имени администратора (один раз).