Angela-lwh/dsh-archive-purge

Adds a Delete action to archived Session rows that permanently removes the Session log directory, the log directories of its subagent Sessions, its Workspace membership and archive mark, its projection-cache record and its spill directory; a Session the Host still holds is staged and finished at the next start or from a cleanup dialog. It also sweeps abandoned blank placeholders, leftover subagent Sessions and orphaned projection-cache records, and keeps blank placeholders out of the @ reference list.