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)

package.json

@@ -13,14 +13,14 @@
       "types": "./dist/index.d.ts",
       "import": "./dist/index.js"
     },
-    "./codex-auth": {
-      "types": "./dist/codex-auth.d.ts",
-      "import": "./dist/codex-auth.js"
+    "./auth": {
+      "types": "./dist/auth.d.ts",
+      "import": "./dist/auth.js"
     },
     "./schema.json": "./dist/oh-my-opencode.schema.json"
   },
   "scripts": {
-    "build": "bun build src/index.ts src/codex-auth.ts --outdir dist --target bun --format esm --external @ast-grep/napi && tsc --emitDeclarationOnly && bun run build:schema",
+    "build": "bun build src/index.ts src/auth.ts --outdir dist --target bun --format esm --external @ast-grep/napi && tsc --emitDeclarationOnly && bun run build:schema",
     "build:schema": "bun run script/build-schema.ts",
     "clean": "rm -rf dist",
     "prepublishOnly": "bun run clean && bun run build",