#72 FTX-72: request_changes semantics (P2)

Policy on_request_changes terminal|hold (default terminal); changes_requested terminal status + feedback_for_agent built in terminalize (UB-ACT-4, EV-DEC-5); hold-mode quorum-unreachable detection in engine (also fixes abstain); actions.change_requests JSONB; reason validation like reject (EV-ACT-3); REST+MCP+Action Web. Phase 7.