Add SunshineR04/dsh-session-manager to Sessions & Messages

[SunshineR04]

Adds SunshineR04/dsh-session-manager — a DeepSeek Harness plugin that manages archived sessions:

• Settings page (Settings → Session Manager): list archived sessions, restore, permanently delete
• Red Delete permanently item in the session context menu
• /sessions slash commands and three agent tools (session_list_archived, session_restore_archived, session_delete_permanently)
• Open sessions delete immediately: bookkeeping + files removed at once, the lingering in-memory summary stays hidden via an archive-set tombstone and is cleaned up at the next restart
• Declares a dsh.bundle manifest + cordis.patch.yml; MIT licensed; 20 unit/render tests