Add Koa framework integration package

Implements @fedify/koa package to integrate Fedify with Koa v2.x and v3.x.

- Add packages/koa with createMiddleware() function
- Support both Koa 2.x and 3.x via peer dependencies
- Convert between Koa context and Web Standards Request/Response
- Handle onNotFound/onNotAcceptable callbacks for routing
- Build for both npm (ESM/CJS) and JSR distribution
- Add examples/koa with basic integration demo
- Update documentation with Koa integration guide
- Update catalog to use Koa 2.16.0 (latest 2.x)

Closes https://github.com/fedify-dev/fedify/issues/454



Co-Authored-By: Claude <noreply@anthropic.com>