feat(brain): add OUTPUT_LANGUAGE reply-language lock

Add an optional OUTPUT_LANGUAGE env var that forces every reply into a
single language. When set, output_language_directive() injects a "respond
only in <language>" instruction (also forbidding other scripts) into the
chat loop's system prompt, next to the existing TTS English-only lock.
Empty (default) keeps the multilingual "reply in the user's language"
behaviour, so upstream is unaffected.

For the Korean-only deployment this also suppresses the occasional trailing
CJK/Hanja fragment qwen2.5:3b leaks on free-form chit-chat.

- system_prompt.py: language-agnostic output_language_directive() helper
- engine.py: read OUTPUT_LANGUAGE, append directive in _build_initial_system_message
- docker-compose.yml + .env.example: document/pass the new var
- docs/llm_contexts.md: note the new gating on the main reply context
- tests: cover unset/set/agnostic/whitespace cases

.env.example

@@ -59,6 +59,12 @@ OLLAMA_CHAT_MODEL=qwen2.5:3b
 OLLAMA_EMBED_MODEL=nomic-embed-text
 WHISPER_MODEL=small
 
+# Lock every reply to one language, e.g. OUTPUT_LANGUAGE=Korean. Leave BLANK to
+# keep the default behaviour of replying in whatever language the user wrote in.
+# A fixed value also suppresses stray characters from other scripts (e.g. the
+# occasional trailing CJK fragment small models leak on free-form chat).
+OUTPUT_LANGUAGE=
+
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