docs(identity): fix sign_ucp_profile docstring example constructor

The example used `UCPSigningKey(**key.public_jwk)`, which raises
TypeError because UCPSigningKey only accepts kid/kty/alg/use/crv/extras
(the JWK `x`/`y` fields belong in extras). Switch to the
`UCPSigningKey.from_jwk(...)` classmethod, matching the runnable
example in examples/signed_ucp_merchant.py.

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

Author: vvillait88

agentscore_commerce/identity/ucp_jwks.py

@@ -260,7 +260,7 @@ def sign_ucp_profile(
 
     Example::
 
-        profile = build_ucp_profile(..., signing_keys=[UCPSigningKey(**key.public_jwk)])
+        profile = build_ucp_profile(..., signing_keys=[UCPSigningKey.from_jwk(key.public_jwk)])
         signed = sign_ucp_profile(profile.to_dict(), signing_key=key.private_key, kid='merchant-2026-05')
     """
     _load_joserfc()