#51 FTX-51: SLA deadlines at creation + delegation caps

tasks.sla_deadline set from step.sla ISO-8601 at instantiation (currently never set -> SLA dead code); delegation end_at required + MAX_DELEGATION_DAYS cap (UB-DEL-2); chain depth from policy (UB-DEL-3); UN-DEL-2 non-member delegate rejected. Phase 2.