#74 FTX-74: escalate + delegate_task actions (P2)

domain/escalation.py execute_escalation_step shared by SLA worker + escalate action (escalated<->in_review, ST-ESC-1); delegate_task: active tenant member (UN-DEL-2), SoD-clean, depth cap; assigned->reassigned->assigned; old assignee tokens revoked (UB-TOK-3); target notified (EV-ACT-6/7). Phase 7.