refactor(auth): rename codex-auth to auth subpath export

- Rename src/codex-auth.ts → src/auth.ts
- Update package.json exports: ./codex-auth → ./auth
- Update build script to include auth.ts

Users can now use oh-my-opencode/auth as OpenAI auth plugin.

🤖 GENERATED WITH ASSISTANCE OF [OhMyOpenCode](https://github.com/code-yeongyu/oh-my-opencode)

src/auth.ts

@@ -0,0 +1 @@
+export { OpenAIAuthPlugin as default } from "opencode-openai-codex-auth";

src/codex-auth.ts

@@ -1 +0,0 @@
-export * from "opencode-openai-codex-auth";