Add AI Model Gateway

[SSC-STUDIO]

Adds AI Model Gateway to the Proxy & API Tools section in both English and Chinese READMEs.

Why it fits:

• It documents a Claude Code gateway path using ANTHROPIC_BASE_URL.
• It is self-hosted and focused on provider fallback, telemetry, request logs, and rollback-safe config changes.
• The project page for this workflow is https://ssc-studio.github.io/Ai-Model-Gateway/claude-code-gateway.html

Disclosure: I maintain AI Model Gateway, so this is a maintainer self-submission. I kept the change to one entry per README and used the existing table/badge format.

Adoption checklist evidence

• Adoption checklist: https://ssc-studio.github.io/Ai-Model-Gateway/llm-gateway-adoption-checklist.html
• Covers fit, install paths, client compatibility, provider fallback, config publish, rollback, quality evidence, and security for maintainers who want a short evaluation path.