dsh-ask-guard: Add a timeout guard for ask_user_question to prevent turns from hanging indefinitely.

# Preface If you have used `ask_user_question` in DeepSeek Harness (DSH), you may have encountered the following scenario: the agent asks a question, and the question card is pushed to the browser via a mux WebSocket in a single push. Once the page is sent to the background, the laptop enters sleep mode, or the connection enters a half-open state, that frame is lost. Without a heartbeat, the client cannot perceive the question's existence, so the tool waits indefinitely. Practical tests have observed hangs lasting over 6 hours, and the only recovery method is manually pressing "Stop". For upstream feedback, see deepseek-ai/deepseek-harness#1554 (discussion). dsh-ask-guard is designed to address this issue.