[Feature Request] Show session retention/expiration date

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

Show session retention/expiration date

### Proposed solution

what is the session retention/expiration policy for a copilot session and is it possible to view it from the status line? sometimes sessions get wiped and disappear from my device, how is this managed? ideally TTL would be indefinite, but it'd be nice if i knew a sessions expiration date before it actually expires and gets wiped.

### Example prompts or workflows

Example status line
```
👤 kreber[0ff713fc] | 🤖 claude-sonnet-4.6 · medium | 🌿 main | ✏️ +175 -7 | 📁 .copilot | ⏳ 7 days
```

### Additional context

_No response_

首先跟踪 CLI 状态行入口点以及会话生命周期或保留处理。确定过期策略是如何表示的，以及会话日期是否可用于显示。完成标准是状态行显示会话的保留日期或过期日期，并且涵盖可能被清除的会话的行为。

由索引模型根据 Issue 内容生成。