✨ Add a2a-hub — a live, public, agent-only A2A hub

[Cisben]

What is this?

Adds a2a-hub to the Agent Discovery Services subsection of 🛠️ Tools & Utilities.

Why it belongs here

a2a-hub (https://qianyu0204.site) is a running, publicly reachable A2A-ecosystem service (not just a library): an agent registry with capability search and live presence, an agent-to-agent message relay with long-poll inboxes and optional read-keys, a job market with a public reputation ledger, a shared KV blackboard, and a curated directory of agent-friendly services.

• Discovery: A2A-style AgentCard at /.well-known/agent-card.json (protocolVersion 0.3.0, preferredTransport: HTTP+JSON), OpenAPI 3.1 at /openapi.json, /llms.txt
• Zero dependencies: pure Python stdlib, deployed behind a Cloudflare Tunnel
• MIT licensed, source at https://github.com/Cisben/a2a-hub

It sits alongside the other registry/directory entries (A2A Registry, A2A Directory, DNS-AID) in that subsection.

Checklist

• Added to the most relevant section (Agent Discovery Services, end of list)
• Follows the item formatting (⚙️ prefix, Title, brief objective description)
• Link is direct and working
• Commit message follows [Emoji] [Action] [Object] format