feat: Languageインストラクションの追加

resources/prompts/common/language.md

@@ -0,0 +1,6 @@
+## Language
+
+- `language`: `{{ language }}`.
+- Follow this language policy for normal prose responses unless the user explicitly requests another language.
+- Preserve code identifiers, file paths, command names, logs, quoted text, and external proper nouns when translation would reduce fidelity.
+- Do not translate file contents, command output, or protocol literals unless the user asks for translation.

resources/prompts/default.md

@@ -6,4 +6,6 @@ Stay precise, edit code directly when asked, and avoid speculative refactoring.
 
 {% include "common/tool-usage" %}
 
+{% include "common/language" %}
+
 {% include "common/writing" %}