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.

docs/llm_contexts.md

@@ -12,7 +12,7 @@ Every distinct LLM call in Jarvis, what feeds it, what consumes it, and how it i
 - **Inputs**:
   - Redacted user query
   - Recent dialogue (last 5 minutes), including in-loop tool-call + tool-role messages from prior replies within the active conversation (tool carryover, `DialogueMemory.record_tool_turn` / `get_recent_turns_with_tools` in [src/jarvis/memory/conversation.py](src/jarvis/memory/conversation.py); per-prompt cap via `cfg.tool_carryover_max_turns` / `tool_carryover_per_entry_chars`; storage cap `_tool_turns_max_storage = 16`; cleared on `stop` signal AND on new-conversation entry; UNTRUSTED WEB EXTRACT fence markers preserved on truncation; both `content` and `tool_calls[*].function.arguments` scrubbed on write)
-  - Unified system prompt from [src/jarvis/system_prompt.py](src/jarvis/system_prompt.py) + ASR note + tool-protocol guidance. When the `OUTPUT_LANGUAGE` env var is set, `output_language_directive()` appends a "respond only in `<language>`" instruction (also forbids other scripts, suppressing trailing CJK leakage on small models); empty (default) keeps the multilingual "reply in the user's language" behaviour. Gated in `_build_initial_system_message()` at [engine.py](src/jarvis/reply/engine.py) alongside the TTS English-only lock.
+  - Unified system prompt from [src/jarvis/system_prompt.py](src/jarvis/system_prompt.py) + ASR note + tool-protocol guidance. Reply language is resolved by `reply_language_directive(OUTPUT_LANGUAGE, cfg.tts_engine)`: an explicit `OUTPUT_LANGUAGE` env lock wins (forces "reply only in `<language>`", also forbidding other scripts so small models stop leaking trailing CJK/Hanja); else a Piper/Chatterbox TTS forces English (English-only voices); else (multilingual TTS, no lock) the assistant replies in the user's own language. The directive is inserted near the FRONT of the guidance list so a small model gives it primacy, and when the lock is set `build_system_prompt()` also rewrites the persona's "in the user's language" clause to the locked language so the persona does not contradict the lock. Gated in `_build_initial_system_message()` at [engine.py](src/jarvis/reply/engine.py).
   - **Warm profile block** (query-agnostic User + Directives excerpt from the knowledge graph, composed by `build_warm_profile()` / `format_warm_profile_block()` in [src/jarvis/memory/graph_ops.py](src/jarvis/memory/graph_ops.py) at Step 3.5 of `reply()`; no LLM call, pure SQLite read; injected unconditionally so personalisation is the default; result cached in `DialogueMemory._hot_cache` under `DialogueMemory.WARM_PROFILE_CACHE_KEY` for the lifetime of the active conversation. Invalidated on `stop`, on new-conversation entry, AND on User/Directives graph mutations via the listener registered in [src/jarvis/daemon.py](src/jarvis/daemon.py) against `register_graph_mutation_listener` in [src/jarvis/memory/graph.py](src/jarvis/memory/graph.py); World-branch writes are ignored)
   - Digested memory enrichment (optional, see #4)
   - Time + location context (re-injected each turn)