Add Sato Hub to x402 Developer Tools & SDKs

[satohubai]

Adds one line to x402 Developer Tools & SDKs (alphabetical, between openai-agents-nano and stipend).

Sato Hub is a free index of onchain-agent tooling. Its x402 slice:

• https://satohub.ai/standards/x402 — tools that implement x402, with the evidence behind each listing
• https://satohub.ai/x402/verified — which URLs tied to x402 listings answer HTTP 402 with a payment payload a client can parse (requests only; no payment is sent)
• https://satohub.ai/x402/facilitators — named x402 facilitators on Base, with the evidence for each address
• https://satohub.ai/mcp — keyless remote MCP server over the same data

The score is a measure of how open, active and verifiable a project is, not a safety or quality grade.

Ran .github/scripts/check_projects_urls.py on the diff locally: no disallowed URLs.