#73 FTX-73: amend_and_approve (P2)

Policy amendments{allowed,fields[json-pointers],schema}; validate allowlist + resulting doc (jsonschema); only on last incomplete primary step (422 AMEND_NOT_LAST_STEP - prevents mid-flight content drift); context_hash immutable + effective_context_hash + outcome.amendments in envelope (EV-ACT-4). Phase 7.