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_

Start by locating the /resume session picker and the code that currently groups sessions by repository and branch. Trace where session last-updated timestamps are available, then determine whether sorting should be the default or an option. Done means recent sessions appear first in descending timestamp order, with the chosen behavior covered by the relevant picker tests.

Written by the indexing model from the issue text.