Add option to sort `/resume` session list by last-updated (most recent first)

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

The `/resume` session picker currently orders sessions by repository/branch grouping rather than recency. This makes it hard to find sessions I worked on recently — a session I used yesterday appeared at position #55, requiring a lot of scrolling.

### Proposed solution

Add a way to sort the `/resume` list by last-updated timestamp, descending (most recently active at the top). Either:
1. Make "last updated, desc" the default, or
2. Provide a toggle/flag (e.g. `/resume --sort=recent`) or a `/settings` option to choose sort order (recent vs. branch-grouped vs. name).

**Why it matters:** Recency is the most common way I look for a session to resume. Scrolling through 50+ branch-grouped entries is slow and error-prone.

### Example prompts or workflows

_No response_

### Additional context

_No response_

首先定位 /resume 工作階段選擇器，以及目前依儲存庫和分支將工作階段分組的程式碼。追蹤工作階段最後更新時間戳記在哪裡可用，接著判斷排序應該是預設行為還是選項。完成的標準是：最近的工作階段依時間戳記降冪排列在前面，且所選擇的行為由相關的 picker 測試涵蓋。

由索引模型根據 Issue 內容生成。