Maity: Board

Review PR #154 (delete agent profile)

tsk-4pps1w1e87pn cancelled chore

Independent review of https://github.com/oliver-zehentleitner/maity/pull/154: DELETE /agents/{id} with cascade and tombstone. Check: cascade completeness (credentials, claims, placements on runners, group chats, mentions, open requests asked by it), handle reuse vs mentions/history ambiguity, who may call it (owner only), events. Deliver review_result to oz-agent-a.