docs(readme): 加 i18n 切换 + 中文版 + nas-auth 引用

- README.md L3 加'English | 简体中文'切换 header
- Choosing an AS / Self-hosted minimal 段:把"Write your own 500 LoC"
  改成显式指向 [nas-auth](https://github.com/ZhengchenTao/nas-auth),
  本 server 开发过程中对接的就是它,作 reference 实现
- 新增 README.zh-CN.md

README.md

@@ -1,5 +1,7 @@
 # gitea-mcp
 
+English | [简体中文](README.zh-CN.md)
+
 Read-only access to a [Gitea](https://about.gitea.com/) instance over
 [MCP (Model Context Protocol)](https://modelcontextprotocol.io/), gated by
 OAuth-issued JWT bearer tokens.
@@ -174,8 +176,7 @@ Public keys are fetched automatically from `<Issuer>/.well-known/openid-configur
 [Keycloak](https://www.keycloak.org), [ZITADEL](https://github.com/zitadel/zitadel), [Logto](https://github.com/logto-io/logto), [Authentik](https://goauthentik.io).
 
 **Self-hosted, minimal** — HS256 mode:
-Write your own ~500 LoC AS that issues HS256 JWTs with the right claims. The
-MCP server's `Jwt__SigningKey__Current` and the AS's signing key must match.
+See [nas-auth](https://github.com/ZhengchenTao/nas-auth) — the reference ~500 LoC AS this server was developed against. Or write your own. The MCP server's `Jwt__SigningKey__Current` and the AS's signing key must match.
 
 **Required AS features regardless of choice:**
 - OAuth 2.1 + PKCE (RFC 7636)