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 会话选择器，以及当前按仓库和分支对会话进行分组的代码。追踪会话最后更新时间戳在哪里可用，然后确定排序应当是默认行为还是一个选项。完成的标准是：最近的会话按时间戳降序排在前面，并且所选择的行为由相关的 picker 测试覆盖。

由索引模型根据 Issue 内容生成。