chore: migrate egg-cors plugin into monorepo

## Migration Task

Migrate [`eggjs/egg-cors`](https://github.com/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