Implement export adapters: Shopify, Cloudflare, and Vercel

[honor2030]

Goal

Add platform-specific export formats to cover the most common redirect destinations.

Acceptance criteria

• Shopify CSV matches the URL Redirects import template (Path, Target).
• Cloudflare CSV matches the documented bulk redirect format.
• Vercel JSON validates against the Vercel config schema.

Task list

• Build Shopify CSV adapter.
• Build Cloudflare bulk redirect CSV adapter.
• Build Vercel vercel.json redirects adapter.
• Write one fixture test per adapter.

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Estimate: 0.5–1 day