chore: migrate koa-override into monorepo

Migration Task

Migrate 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