feat(list): add openclaw-a2a-gateway

[AliceLJY]

Resource name: win4r/openclaw-a2a-gateway
Resource URL: https://github.com/win4r/openclaw-a2a-gateway
Suggested section: Plugins and Integrations
Why it should be included: A2A (Agent-to-Agent) v0.3.0 protocol plugin for OpenClaw with bio-inspired routing, mDNS discovery, circuit breaker, and DevTools CLI (378 stars, 486 tests). Bridges OpenClaw to the Google-initiated A2A ecosystem, enabling agent-to-agent interoperability.

• I checked that this resource is not already listed.
• I reviewed CONTRIBUTING.md formatting and A-Z ordering rules.