chore: migrate koa-override into monorepo

## Migration Task

Migrate [`eggjs/koa-override`](https://github.com/eggjs/koa-override) into the egg monorepo.

## Source

- Repo: https://github.com/eggjs/koa-override
- Package: `koa-override`

## Target

- Path: `packages/koa-override/`
- Package name: `@eggjs/koa-override` (or keep `koa-override`)

## Checklist

- [ ] Move source code to `packages/koa-override/src/`
- [ ] Update package.json (type: module, exports, publishConfig)
- [ ] Add minimal `tsconfig.json` extending root config
- [ ] Add `vitest.config.ts` and migrate tests
- [ ] Update root `tsconfig.json` references
- [ ] Update internal dependencies to use `workspace:*`
- [ ] Verify build and tests pass
- [ ] Archive the original repo after migration