chore: migrate egg-cors plugin into monorepo #5799

Migration Task

Migrate eggjs/egg-cors into the egg monorepo.

Source

• Repo: https://github.com/eggjs/egg-cors
• Package: egg-cors

Target

• Path: plugins/cors/
• Package name: @eggjs/cors (or keep egg-cors)

Checklist

• Move source code to plugins/cors/src/
• Update package.json (type: module, exports, publishConfig, peerDependencies: egg)
• Add minimal tsconfig.json extending root config
• Add vitest.config.ts and migrate tests
• Add src/types.ts for module augmentation (extend EggAppConfig)
• Update root tsconfig.json references
• Update internal dependencies to use workspace:*
• Verify build and tests pass
• Archive the original repo after migration