Improve organization of background tasks and artifacts

## Problem

Background tasks and artifacts become difficult to manage as their number grows. The current lists do not provide enough ways to find or organize older items.

## Proposed improvements

- Sort or group items by creation time
- Group items by date or session
- Pin important artifacts
- Add labels or color coding for easier identification

These controls should make large task and artifact lists easier to scan and manage.

Start by reviewing the current background-task and artifact list experiences in the app. Clarify which organization capability is in scope—sorting, grouping, pinning, or labels—and define how users will find older items and recognize that the work is complete. No files, entry points, or tests are named in the issue.

索引モデルが issue の本文から書いたものです。