feat(create-nextrush): scaffold production-ready project foundation

## Summary

Improve the default project scaffolding generated by `create-nextrush` so new projects start with a more complete foundation, not just source folders.

## Motivation

The current scaffolder already does a strong job generating the application structure, but users still need to add common project-level files manually after scaffolding.

## Proposed changes

Generate these by default:

- `.env`
- `.env.example`
- `.editorconfig`
- `.gitignore`
- `README.md`
- `package.json`
- `tsconfig.json`
- `src/env.d.ts`

Add or improve the generated `package.json` so it includes useful defaults such as:

- `engines.node` set to `>=22`
- standard scripts like `dev`, `build`, `test`, `lint`, `typecheck`
- clear `private`, `type`, and package manager metadata

Provide a centralized environment setup so generated apps avoid reading `process.env` everywhere.

## Environment example

`.env`

```env
PORT=3000
HOST=0.0.0.0
NODE_ENV=development
```

`.env.example`

```env
PORT=
HOST=
NODE_ENV=
```

## Why

This would make the generated project feel much more production-ready from day one and reduce repetitive manual setup.

## Scope

Keep the default scaffold lightweight, but make the baseline closer to what developers typically expect in a real project.

## Non-goals

- Do not add framework configuration files unless they clearly remove complexity.
- Do not make the default scaffold too heavy.
- Do not introduce arbitrary custom template/plugin support.

## Acceptance criteria

- A freshly generated project includes the common root-level project files.
- Environment configuration is centralized.
- The default generated app still stays simple and testable.
- Existing scaffold styles continue to work.