fix: harden Korean-only output lock (front+end, explicit script ban)

qwen2.5:7b leaked Chinese/Cyrillic mid-reply despite the OUTPUT_LANGUAGE
lock, which was buried mid-prompt. Repeat the lock at the END of the system
prompt (recency) and ban specific foreign scripts explicitly.

src/jarvis/system_prompt.py

@@ -133,8 +133,12 @@ def output_language_directive(language: Optional[str]) -> Optional[str]:
         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."
+        f"matching or using the user's language. Do NOT output a single Chinese/"
+        f"Hanja character, Japanese kana, Cyrillic letter, Arabic letter, or any "
+        f"other non-{lang} script anywhere in the reply — not even one word or "
+        f"clause. If a {lang} word exists, use it; never substitute or append a "
+        f"foreign-language equivalent. (Numerals and unavoidable proper-noun "
+        f"brand names are fine.)"
     )