[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_

首先追蹤 CLI 狀態列進入點，以及工作階段生命週期或保留處理。確定到期原則如何表示，以及工作階段日期是否可供顯示。完成標準是狀態列顯示工作階段的保留日期或到期日期，並且涵蓋可能被清除之工作階段的行為。

由索引模型根據 Issue 內容生成。