Revert "feat: replace MeloTTS with Coqui XTTS-v2 natural Korean voice"

This reverts commit 39a0944105.

tests/test_system_prompt.py

@@ -101,14 +101,6 @@ class TestReplyLanguageDirective:
         # user's own language, so no directive.
         assert reply_language_directive(None, "melo") is None
 
-    def test_xtts_is_multilingual(self):
-        # XTTS-v2 (the Korean voice) is not English-only: no lock → free, and a
-        # lock is honoured (not overridden to English).
-        assert reply_language_directive(None, "xtts") is None
-        directive = reply_language_directive("Korean", "xtts")
-        assert directive is not None and "Korean" in directive
-        assert directive != ENGLISH_ONLY_DIRECTIVE
-
     def test_unknown_tts_defaults_to_english_only(self):
         # Preserves the original getattr(cfg, 'tts_engine', 'piper') default:
         # an unknown/missing engine is treated conservatively as English-only.