# description
The §Y change-notification webhook envelope a consumer POSTs to a broker-configured receiver ([OM-HOOK-002]). Signed per §Y (OpenOM-Signature); this schema validates the JSON body a receiver parses. Human-readable mirror is spec Part II §5b.

# properties.id.description
stable event id (== OpenOM-Event-Id, stable across retries)

# properties.payloadHash.description
The §C integrity hash of `payload`. JSON Schema cannot bind it to `payload`, so a receiver MUST recompute and compare (verifyEnvelopePayloadHash) after verifying the signature - a schema-valid envelope can still carry a mismatched hash ([OM-HOOK], #120).