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.

src/jarvis/system_prompt.py

@@ -81,14 +81,26 @@ _SYSTEM_PROMPT_TEMPLATE: str = (
 )
 
 
-def build_system_prompt(assistant_name: str = "Jarvis") -> str:
+def build_system_prompt(
+    assistant_name: str = "Jarvis", output_language: Optional[str] = None
+) -> str:
     """Render the persona prompt with the configured assistant name.
 
     The name comes from the user's wake word (capitalised); defaults to
     "Jarvis" when no config is available (tests, eval harnesses).
+
+    When ``output_language`` is set (a single-language deployment), the
+    persona's "reply in the user's language" clause is rewritten to that
+    language so the persona does not contradict the OUTPUT_LANGUAGE lock — a
+    small model otherwise honours the persona's instruction to mirror the
+    query language and leaks the other language back in.
     """
     name = (assistant_name or "Jarvis").strip() or "Jarvis"
-    return _SYSTEM_PROMPT_TEMPLATE.format(name=name)
+    prompt = _SYSTEM_PROMPT_TEMPLATE.format(name=name)
+    lang = (output_language or "").strip()
+    if lang:
+        prompt = prompt.replace("in the user's language", f"in {lang}")
+    return prompt
 
 
 def output_language_directive(language: Optional[str]) -> Optional[str]:
@@ -108,7 +120,41 @@ def output_language_directive(language: Optional[str]) -> Optional[str]:
     if not lang:
         return None
     return (
-        f"Always respond only in {lang}, regardless of the language the user "
-        f"writes in. Do not mix in words, characters, or punctuation from any "
-        f"other language or script."
+        f"CRITICAL OUTPUT RULE: write your ENTIRE reply only in {lang}. Even if "
+        f"the user writes in English or any other language, you must still reply "
+        f"only in {lang}. This rule overrides every other instruction about "
+        f"matching or using the user's language. Never mix in words, characters, "
+        f"or punctuation from any other language or script."
     )
+
+
+# TTS engines that can only synthesise English. Replying in another language
+# with one of these produces garbled audio, so those deployments force English.
+_TTS_ENGLISH_ONLY = frozenset({"piper", "chatterbox"})
+
+# Kept verbatim for backward compatibility with anything asserting on the wording.
+ENGLISH_ONLY_DIRECTIVE = (
+    "Always respond in English regardless of the language the user speaks in."
+)
+
+
+def reply_language_directive(
+    output_language: Optional[str], tts_engine: Optional[str]
+) -> Optional[str]:
+    """Resolve the reply-language instruction for the chat loop, or None.
+
+    Precedence:
+    1. An explicit ``output_language`` lock wins — the deployment serves a
+       single language and owns a TTS voice that can speak it (e.g. Korean
+       MeloTTS). This intentionally overrides the English-only fallback.
+    2. Otherwise, a Piper/Chatterbox TTS engine can only synthesise English,
+       so force English to avoid garbled audio.
+    3. Otherwise (multilingual TTS, no lock) → None: the assistant replies in
+       the user's own language.
+    """
+    forced = output_language_directive(output_language)
+    if forced:
+        return forced
+    if (tts_engine or "piper").strip().lower() in _TTS_ENGLISH_ONLY:
+        return ENGLISH_ONLY_DIRECTIVE
+    return None