docs(PR34): remove SPA and SettlementIntent from reference flows (#34)

Update fleet-ev-reference-flow.md and human-agent-reference-flow.md to
align with the revised MPCP protocol where the Trust Gateway is mandatory
and the authorization chain ends at SBA.

- Remove SignedPaymentAuthorization (SPA) sections, fields, and timeline steps
- Remove SettlementIntent sections and bundle examples
- Add Trust Gateway as explicit actor in both flows
- Update artifact tables, storage matrices, and audit bundles
- Update timeline steps to reflect SBA → Trust Gateway → XRPL settlement
- Update Full Bundle Examples (no SPA/settlementIntent; add budgetEscrowRef/authorizedGateway to PolicyGrant)
- Update failure scenarios to reference SBA instead of SPA
- Reflect that cumulative budget is enforced by Trust Gateway, not agent self-reporting

Co-authored-by: NAOR YUVAL <naoryuval@NAORs-MacBook-Air.local>
Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 3 people