feat(brain): make OUTPUT_LANGUAGE lock robust on small models

Harden the reply-language lock so qwen2.5:3b reliably stays in the locked
language instead of leaking the query language back in:

- reply_language_directive(): single resolver with clear precedence —
  explicit OUTPUT_LANGUAGE lock wins over the Piper/Chatterbox English-only
  fallback (this deployment's actual TTS is Korean MeloTTS, so the legacy
  English lock was both wrong and contradicting the Korean lock).
- Stronger, override-explicit directive wording, inserted near the FRONT of
  the system prompt so a small model gives it primacy over the persona.
- build_system_prompt(output_language=...): rewrite the persona's "in the
  user's language" clause to the locked language so the persona stops
  fighting the lock.
- docs/llm_contexts.md: document the resolver, precedence, and placement.

Live-verified on the running brain (qwen2.5:3b): Korean voice-style input
and a cold English query both return fully Korean replies with no CJK/Hanja
leak. Tests cover unset/set/agnostic/whitespace + precedence + persona rewrite.

tests/test_system_prompt.py

@@ -5,7 +5,12 @@ wake word to e.g. "Friday" produces a butler named Friday, not one still
 hardcoded to Jarvis.
 """
 
-from jarvis.system_prompt import build_system_prompt, output_language_directive
+from jarvis.system_prompt import (
+    build_system_prompt,
+    output_language_directive,
+    reply_language_directive,
+    ENGLISH_ONLY_DIRECTIVE,
+)
 
 
 class TestBuildSystemPrompt:
@@ -27,6 +32,17 @@ class TestBuildSystemPrompt:
         assert "named Jarvis" in build_system_prompt("   ")
         assert "named Jarvis" in build_system_prompt(None)  # type: ignore[arg-type]
 
+    def test_default_keeps_user_language_clause(self):
+        # Without a lock, the persona still mirrors the user's language.
+        assert "in the user's language" in build_system_prompt("Jarvis")
+
+    def test_language_lock_rewrites_user_language_clause(self):
+        # With a lock, the contradicting "user's language" clause is rewritten
+        # so the persona does not fight the OUTPUT_LANGUAGE directive.
+        prompt = build_system_prompt("Jarvis", "Korean")
+        assert "in the user's language" not in prompt
+        assert "in Korean" in prompt
+
 
 class TestOutputLanguageDirective:
     """A deployment may lock replies to a single language via OUTPUT_LANGUAGE.
@@ -58,3 +74,37 @@ class TestOutputLanguageDirective:
         assert directive is not None
         assert "Korean" in directive
         assert "  Korean" not in directive
+
+
+class TestReplyLanguageDirective:
+    """Precedence: explicit OUTPUT_LANGUAGE lock > English-only TTS > free.
+
+    The lock must override the Piper/Chatterbox English fallback, because a
+    deployment that sets OUTPUT_LANGUAGE (e.g. Korean) also runs a TTS voice
+    that can speak it. Without this, the English lock and the Korean lock
+    contradict each other and the model reverts to English.
+    """
+
+    def test_lock_overrides_english_only_tts(self):
+        directive = reply_language_directive("Korean", "piper")
+        assert directive is not None
+        assert "Korean" in directive
+        assert directive != ENGLISH_ONLY_DIRECTIVE
+
+    def test_english_only_tts_forces_english_without_lock(self):
+        assert reply_language_directive(None, "piper") == ENGLISH_ONLY_DIRECTIVE
+        assert reply_language_directive("", "chatterbox") == ENGLISH_ONLY_DIRECTIVE
+
+    def test_no_lock_multilingual_tts_is_free(self):
+        # A non-English-only engine (e.g. melo) with no lock → reply in the
+        # user's own language, so no directive.
+        assert reply_language_directive(None, "melo") is None
+
+    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.
+        assert reply_language_directive(None, None) == ENGLISH_ONLY_DIRECTIVE
+
+    def test_lock_wins_even_with_multilingual_tts(self):
+        directive = reply_language_directive("Korean", "melo")
+        assert directive is not None and "Korean" in directive