dsh-session-activity: Activity-Prioritized Session Organization in the DSH Web Sidebar

The `dsh-session-activity` plugin automatically categorizes sessions in the DeepSeek Harness (DSH) sidebar into four tiers—Hot, Warm, Cool, and Cold—based on `Last updated` timestamps to reduce visual clutter. It supports dynamic sorting, a Focus mode for highlighting active sessions, and preserves native operations. Users can configure time thresholds and refresh intervals in `$DSH_HOME/session-organizer.json`, making it suitable for developers managing multiple concurrent sessions over extended periods.