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.

まず GitHub Copilot CLI のトランスクリプトのレンダリングとセッション再開のフローを追跡し、次にメッセージの作成時刻を利用できる場所を特定します。タイムスタンプの表示方法と時間間隔の区切り文字をどのように設定するかを決める前に、CLI フラグまたは設定がどのように定義されているかを確認します。完了条件は、再開された会話とスケジュールされた会話で、正しい時刻のターンが表示され、デフォルトのターミナル出力が不必要に煩雑にならないことです。

索引モデルが issue の本文から書いたものです。