feat: add create-vinext-app scaffolding CLI

[Divkix]

Summary

Add create-vinext-app — a standalone CLI package that scaffolds new vinext projects via npm create vinext-app@latest.

Motivation

Currently new projects require npm create next-app@latest → vinext init → manual wrangler/worker setup (8-12 steps). This should be one command.

Scope

• New packages/create-vinext-app/ package
• 2 templates: App Router, Pages Router (both with Cloudflare Workers)
• Interactive prompts via @clack/prompts
• --yes flag for non-interactive CI usage
• CI job to test scaffolding
• Publish workflow integration
• README, CLAUDE.md, CONTRIBUTING.md updates

Acceptance Criteria

• npm create vinext-app@latest my-app --yes scaffolds a working project
• cd my-app && bun run dev starts successfully with HTTP 200
• App Router and Pages Router templates both work
• All tests pass (unit, integration, e2e)
• CI workflow tests scaffolding on ubuntu + windows
• Publish workflow publishes to npm alongside vinext
• README updated with new instructions

[james-elicx]

N.B. I've claimed the package name for now - can transfer ownership later if needed.

I would agree that it's reasonable for vinext to have it's own create-vinext-app package separate to create-cloudflare, given that non-cloudflare builds are supported. That and aligning with what Next.js has. (happy to be challenged on that though!)