feat(create-nextrush): add scaffold presets for project templates

## Summary

Add scaffold presets to `create-nextrush` so users can choose a project foundation based on their needs instead of always getting the same baseline.

## Why this matters

As the scaffolder grows, the default experience should stay simple, but more advanced users will need a clearer way to opt into a richer setup. Presets make the generator easier to understand, easier to extend, and easier to document.

## Proposal

Introduce a preset selection layer in the scaffolder. Example presets:

- `minimal`
- `api`
- `production`
- `microservice`

Each preset should map to a defined set of generated files, dependencies, scripts, and optional features.

## Example preset behavior

### Minimal

Best for small projects and quick starts.

Includes only the essentials:

- `package.json`
- `tsconfig.json`
- `README.md`
- `.gitignore`
- basic source layout

### API

Best for typical backend APIs.

Includes:

- environment scaffolding
- linting configuration
- testing configuration
- API-focused source structure
- useful scripts

### Production

Best for teams that want deployment-ready defaults.

Includes:

- everything in `api`
- Docker support
- CI workflow
- editor defaults
- stronger metadata in `package.json`

### Microservice

Best for service-oriented systems.

Includes:

- health checks
- observability-friendly structure
- runtime and deployment defaults
- container-friendly setup

## Acceptance criteria

- Users can select a preset during scaffolding
- Each preset produces a clearly defined file set
- Presets remain curated rather than becoming arbitrary template plugins
- The default experience stays lightweight
- Documentation explains what each preset includes

## Non-goals

- Do not allow arbitrary user-defined scaffold plugins
- Do not make the CLI overly complex
- Do not force advanced files into the minimal preset

## Notes

This issue should focus on the preset abstraction itself. Individual files like env, lint, Docker, or CI should continue to be tracked in separate implementation issues.