docs(README): promote example imports to top-level where re-exported

- Quick-start: RailSpec types + validate_shipping_against_policy via top-level
- Payment helpers section: extract_payment_signer via top-level

Remaining /payment + /identity.policy imports are submodule-only helpers
(build_payment_directive, networks, build_x402_accepts_for_402, etc.).

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: vvillait88

README.md

@@ -84,10 +84,10 @@ async def purchase(request: Request, assess=Depends(get_agentscore_data)):
 from fastapi import FastAPI, Request
 from agentscore_commerce import (
     Checkout, CheckoutGateConfig, DiscoveryProbeConfig, PricingResult, pricing_result,
+    SolanaMppRailSpec, StripeRailSpec, TempoRailSpec, X402BaseRailSpec,
+    validate_shipping_against_policy,
 )
 from agentscore_commerce.discovery import default_a2a_services
-from agentscore_commerce.identity.policy import validate_shipping_against_policy
-from agentscore_commerce.payment import TempoRailSpec, X402BaseRailSpec, SolanaMppRailSpec, StripeRailSpec
 
 app = FastAPI()
 
@@ -161,11 +161,11 @@ The 402 body Checkout emits auto-attaches `identity_mode` + `required_signer` +
 ## Payment helpers
 
 ```python
+from agentscore_commerce import extract_payment_signer
 from agentscore_commerce.payment import (
     BuildPaymentDirectiveInput,
     PaymentDirectiveInput,
     build_payment_directive,
-    extract_payment_signer,
     networks,
     payment_directive,
     www_authenticate_header,