docs(readme): update A2A snippet for spec-compliant card shape

The README's identity-publishing example for build_a2a_agent_card used the
pre-refactor shape (capabilities=A2AAgentCardCapabilities(...) + data=
assess_result + no version / description / skills). Update to the
spec-compliant inputs (top-level description, version, skills as
A2AAgentSkill objects, extensions inside capabilities via the helper).

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

Author: vvillait88

README.md

@@ -185,17 +185,33 @@ from agentscore_commerce.identity import (
     UCPServiceBinding,
     UCPSigningKey,
     UCPPaymentHandlerBinding,
-    A2AAgentCardCapabilities,
+    A2AAgentSkill,
     build_a2a_agent_card,
     build_ucp_profile,
     ucp_a2a_extension,
 )
 
 # Google A2A v1.0 Signed Agent Card. Publish at /.well-known/agent-card.json.
-# Per UCP §A2A binding the card MUST declare the canonical UCP extension URI;
-# pass `ucp_a2a_extension()` with empty capabilities until you bind formal UCP
-# capabilities (dev.ucp.shopping.checkout, etc.).
-card = build_a2a_agent_card(name="My Service", url=base_url, capabilities=A2AAgentCardCapabilities(...), extensions=[ucp_a2a_extension()], data=assess_result)
+# Per UCP §A2A binding the card MUST declare the canonical UCP extension URI in
+# `capabilities.extensions[]`; pass `ucp_a2a_extension()` with empty capabilities
+# until you bind formal UCP capabilities (dev.ucp.shopping.checkout, etc.).
+# Skills are top-level AgentSkill objects; identity claims live in a separate
+# AgentCardSignature (RFC 7515 JWS) wrapping the serialized card.
+card = build_a2a_agent_card(
+    name="My Service",
+    description="Buy products via agent payments.",
+    url=base_url,
+    version="1.0.0",
+    skills=[
+        A2AAgentSkill(
+            id="purchase",
+            name="Purchase",
+            description="Buy products via agent payments.",
+            tags=["commerce", "payment"],
+        ),
+    ],
+    extensions=[ucp_a2a_extension()],
+)
 
 # Google Universal Commerce Protocol. Publish at /.well-known/ucp.
 # Output shape: {"ucp": {"version", "services", "capabilities",