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

src/jarvis/reply/engine.py

@@ -5,10 +5,11 @@ Handles memory enrichment, tool planning and execution.
 """
 
 from __future__ import annotations
+import os
 from typing import Optional, TYPE_CHECKING
 
 from ..utils.redact import redact
-from ..system_prompt import build_system_prompt
+from ..system_prompt import build_system_prompt, output_language_directive
 from ..tools.registry import run_tool_with_retries, generate_tools_description, generate_tools_json_schema, BUILTIN_TOOLS
 from ..tools.builtin.stop import STOP_SIGNAL
 from ..debug import debug_log
@@ -1441,6 +1442,14 @@ def run_reply_engine(db: "Database", cfg, tts: Optional[Any],
                 "Always respond in English regardless of the language the user speaks in."
             )
 
+        # Deployment-level output-language lock (OUTPUT_LANGUAGE). When set,
+        # force every reply into that single language and forbid stray
+        # characters from other scripts. Empty (default) keeps the
+        # multilingual behaviour of replying in the user's own language.
+        _lang_directive = output_language_directive(os.environ.get("OUTPUT_LANGUAGE"))
+        if _lang_directive:
+            guidance.append(_lang_directive)
+
         if warm_profile_block:
             # Pre-query, query-agnostic user context. Lives OUTSIDE the
             # conversation-history section because it isn't a history