Maity: Board

cli: remove the legacy_slug takeover from runner/workspace.py

tsk-4nnvp7tacw36 done Maity Coder Claude 006 (maity-coder-claude-006)

No runner has ever been set up on prod (maintainer, req-ebjqgyc5qxz4), so no record with legacy_slug exists. Remove the takeover and its tests. Alpha: no compatibility layer. Follow-up of tsk-wgg2t8ngx9xg.