docs(PR36): remove SPA/SettlementIntent from sequence diagrams and reference flow note (#36)

* docs: remove SPA/SettlementIntent from reference flows and sequence diagrams

- fleet-ev-reference-flow.md: drop explanatory note mentioning SPA/SettlementIntent
- reference-flow-sequence.svg: steps 7–8 now SBA signing + revocation check; bundle
  label SBA·PolicyGrant; verification removes SPA lines; settlement arrow from Trust
  Gateway column; step 13 anchors policyHash (not intentHash)
- human-agent-reference-flow-sequence.svg: same treatment — SBA replaces SPA in steps
  7–8 and bundle label; XRPL payment arrow from Trust Gateway side; SBA refused (not SPA)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

* docs: link Trust Gateway and XRPL Settlement in homepage chain

Both terms now link to existing spec pages (actors.md#trust-gateway, rails.md)
so the authorization chain is fully navigable.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

---------

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

Author: 3 people

docs/architecture/fleet-ev-reference-flow.md

@@ -896,7 +896,6 @@ This bundle illustrates the canonical MPCP artifact structure:
 - `sba.authorization.maxAmountMinor` = `"780"` (this payment only, in USD cents); the Trust Gateway enforces cumulative ≤ `policyGrant.budgetMinor`
 - `chargingQuote` is not an MPCP artifact but is operationally important — the operator must verify that the SBA `maxAmountMinor` and `destinationAllowlist` cover the quoted amount and destination
 - `settlement.txHash` is the XRPL transaction submitted by the Trust Gateway; the `mpcp/grant-id` memo links the on-chain payment to the grant for audit
-- There is no SPA or SettlementIntent — the authorization chain ends at the SBA; the Trust Gateway handles all settlement parameters
 
 In a production deployment, the exact bundle shape may vary, but it should preserve the same key property:
 

docs/architecture/human-agent-reference-flow-sequence.svg

@@ -10,7 +10,7 @@ Autonomous vehicles, AI agents, and machines increasingly pay for real-world ser
 
 A machine payment is accepted only if the recipient can verify a chain of signed artifacts:
 
-**[Fleet Policy](protocol/FleetPolicyAuthorization.md) → [PolicyGrant](protocol/PolicyGrant.md) → [SignedBudgetAuthorization](protocol/SignedBudgetAuthorization.md) → Trust Gateway → XRPL Settlement**
+**[Fleet Policy](protocol/FleetPolicyAuthorization.md) → [PolicyGrant](protocol/PolicyGrant.md) → [SignedBudgetAuthorization](protocol/SignedBudgetAuthorization.md) → [Trust Gateway](architecture/actors.md#trust-gateway) → [XRPL Settlement](protocol/rails.md)**
 
 Each step narrows what the machine is allowed to do.