Show message timestamps in GitHub Copilot CLI conversations

### Describe the feature or problem you'd like to solve

GitHub Copilot CLI doesn't show when each prompt or response occurred, making it hard to tell when something was asked or answered and whether a session was interrupted.

### Proposed solution

Display a timestamp for each turn (user prompt and Copilot response) in the CLI transcript.

• Show a compact local-time stamp per message (e.g.,  [09:47]  or  [2026-06-25 09:47] ), respecting the user's timezone.
• Optionally insert a time-gap separator when a significant interval elapses between turns (e.g.,  — 3 hours later — ), so interruptions and resumed sessions are obvious when scrolling back.
• Make it configurable (on/off, relative vs. absolute, time vs. date+time) via a CLI flag or setting.
• Include timestamps when a session is resumed so the boundary between old and new turns is clear.

### Example prompts or workflows

• Resumed session: I run a task in the morning, step away, and resume the CLI hours later — timestamps immediately show the gap and that the last answer is stale.
• Scheduled/recurring prompts: A scheduled prompt posts updates into the session; timestamps confirm exactly when each run executed and replied.
• Operational debugging: Correlating Copilot CLI suggestions with deployment and log timestamps to reconstruct an accurate timeline.

### Additional context

• The CLI already runs long-lived and resumable sessions where timing context matters most, yet that information isn't surfaced.
• A compact, optional timestamp keeps the terminal output clean while preserving precision when needed.
• This is primarily a UX enhancement; the timing data already exists at message creation time.

先追蹤 GitHub Copilot CLI 的 transcript 渲染和工作階段恢復流程，接著找出訊息建立時間可在哪裡取得。在決定應如何設定時間戳顯示和時間間隔分隔符之前，先檢查 CLI 旗標或設定的定義方式。完成的標準是：恢復的對話和已排程的對話會顯示時間正確的回合，同時不會讓預設終端機輸出變得不必要地嘈雜。

由索引模型根據 Issue 內容生成。