tweak(session title): change prompt to have the response with user language (#9847)

packages/opencode/src/agent/prompt/title.txt

@@ -12,6 +12,7 @@ Your output must be:
 </task>
 
 <rules>
+- you MUST use the same language as the user message you are summarizing
 - Title must be grammatically correct and read naturally - no word salad
 - Never include tool names in the title (e.g. "read tool", "bash tool", "edit tool")
 - Focus on the main topic or question the user needs to retrieve