Sidebar enhancements

### Preflight Checklist

- [x] I have searched [existing requests](https://github.com/anthropics/claude-code/issues?q=is%3Aissue%20label%3Aenhancement) and this feature hasn't been requested yet
- [x] This is a single feature request (not multiple features)

### Problem Statement

As work piles up the list of sessions gets longer and it becomes harder to find old sessions that you want to reference.

### Proposed Solution

The sidebar in the desktop app could use two enhancements:
1. The ability to sort sessions by name.
2. The ability to organize sessions into groups.

### Alternative Solutions

_No response_

### Priority

Critical - Blocking my work

### Feature Category

CLI commands and flags

### Use Case Example

_No response_

### Additional Context

_No response_

The issue names no files, tests, or entry points. Start by locating the desktop app's sidebar and session-list implementation, then clarify the interaction and storage expectations for sorting and grouping; done means both enhancements are available and their behavior is covered by the relevant tests.

索引モデルが issue の本文から書いたものです。