fix: persona uses settings output_language, matching the reply directive

The persona prompt was built from the raw OUTPUT_LANGUAGE env while the
reply-language directive read the settings-web UI value (config JSON), so
changing the language in the settings page was honoured by the directive
but ignored by the persona, leaving them contradicting each other.

Add _resolve_output_language() as the single source of truth (config wins
over env) and feed the same resolved value to both build_system_prompt()
and reply_language_directive(). Update docs/llm_contexts.md to match.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/test_output_language_resolution.py

@@ -0,0 +1,74 @@
+"""The locked reply language must have a single source of truth.
+
+Regression: the persona prompt was built from the raw ``OUTPUT_LANGUAGE`` env
+while the reply-language directive read the settings-UI value (config JSON).
+Changing the language in the settings page rewrote the directive but left the
+persona contradicting it. ``_resolve_output_language`` is now the one resolver
+both call sites use, so they cannot diverge.
+"""
+
+import pytest
+
+
+@pytest.mark.unit
+def test_settings_value_wins_over_env(monkeypatch, tmp_path):
+    from jarvis.reply.engine import _resolve_output_language
+
+    cfg_path = tmp_path / "config.json"
+    cfg_path.write_text('{"output_language": "Korean"}', encoding="utf-8")
+    monkeypatch.setenv("JARVIS_CONFIG_PATH", str(cfg_path))
+    monkeypatch.setenv("OUTPUT_LANGUAGE", "English")
+
+    # The settings page value must take effect over the compose env default.
+    assert _resolve_output_language() == "Korean"
+
+
+@pytest.mark.unit
+def test_env_used_when_settings_absent(monkeypatch, tmp_path):
+    from jarvis.reply.engine import _resolve_output_language
+
+    cfg_path = tmp_path / "config.json"
+    cfg_path.write_text("{}", encoding="utf-8")
+    monkeypatch.setenv("JARVIS_CONFIG_PATH", str(cfg_path))
+    monkeypatch.setenv("OUTPUT_LANGUAGE", "English")
+
+    assert _resolve_output_language() == "English"
+
+
+@pytest.mark.unit
+def test_unset_when_neither_configured(monkeypatch, tmp_path):
+    from jarvis.reply.engine import _resolve_output_language
+
+    cfg_path = tmp_path / "config.json"
+    cfg_path.write_text("{}", encoding="utf-8")
+    monkeypatch.setenv("JARVIS_CONFIG_PATH", str(cfg_path))
+    monkeypatch.delenv("OUTPUT_LANGUAGE", raising=False)
+
+    # Empty string or None both mean "no lock" downstream; normalise the check.
+    assert not _resolve_output_language()
+
+
+@pytest.mark.unit
+def test_persona_and_directive_agree_on_settings_value(monkeypatch, tmp_path):
+    """End-to-end: the same resolved value feeds the persona and the directive,
+    so a settings-UI language can't be honoured by one and ignored by the other.
+    """
+    from jarvis.reply.engine import _resolve_output_language
+    from jarvis.system_prompt import build_system_prompt, reply_language_directive
+
+    cfg_path = tmp_path / "config.json"
+    cfg_path.write_text('{"output_language": "Korean"}', encoding="utf-8")
+    monkeypatch.setenv("JARVIS_CONFIG_PATH", str(cfg_path))
+    monkeypatch.setenv("OUTPUT_LANGUAGE", "English")
+
+    lang = _resolve_output_language()
+    persona = build_system_prompt("Jarvis", lang)
+    directive = reply_language_directive(lang, "melo")
+
+    # Persona's user-language clause is rewritten to Korean, not English...
+    assert "in Korean" in persona
+    assert "in English" not in persona
+    # ...and the directive locks to the same Korean. (The directive may name
+    # English as a counter-example - "even if the user writes in English" - so
+    # we assert the lock target, not the mere absence of the word "English".)
+    assert directive is not None and "Korean" in directive