Retires the shared prefix. Live plans move to the new prefix keeping their
running number; finished and archived files keep the old prefix as
historical record, per the ADR-007 option 2 migration ruling.
Hub UUIDs are unchanged, so reconciliation matches on
state_hub_workstream_id rather than filename and will update the slug in
place instead of registering duplicates.
Assigned in RMGR-WP-0004-T09.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
