feat: trim integration pages from 75 to 25 + migrate to MDX

- Add dedicatedPage flag to integration registry (25 true, 50 false)
- Delete hardcoded nextjs/react/vue/wordpress route pages (wrong metadata)
- Hub page routes non-dedicated integrations to /integrations/script-tag
- Add 301 redirects for 50 removed slugs → /integrations/script-tag
- Migrate guide content from TSX to MDX (content/integrations/*.mdx)
- Add gray-matter, next-mdx-remote, remark-gfm dependencies
- Add content loader (lib/integration-content.ts) matching ciphera-website pattern
- Add prebuild script for integration guide index generation
- Sitemap reduced from 83 to 35 URLs with real lastmod dates
- Remove seoDescription from registry (now in MDX frontmatter)

content/integrations/shopify.mdx

@@ -0,0 +1,32 @@
+---
+title: "Shopify"
+description: "Add Pulse privacy-first analytics to your Shopify store via the theme editor."
+category: "ecommerce"
+brandColor: "#7AB55C"
+officialUrl: "https://shopify.dev/docs"
+relatedIds: ["woocommerce", "bigcommerce", "prestashop"]
+date: "2026-03-28"
+---
+
+Add the Pulse script via the Shopify theme editor — no app needed.
+
+---
+
+## Method 1: Edit theme code
+
+Go to **Online Store -> Themes -> Edit code** and open `layout/theme.liquid`. Add the Pulse script before the closing `</head>` tag.
+
+<CodeBlock filename="layout/theme.liquid">{`<!-- Add before </head> -->
+<script
+  defer
+  data-domain="your-site.com"
+  src="https://pulse.ciphera.net/script.js"
+></script>`}</CodeBlock>
+
+## Method 2: Shopify Plus — Customer Events
+
+If you're on Shopify Plus, you can add the Pulse script via **Settings -> Customer Events -> Custom Pixels**.
+
+Use your custom domain or `.myshopify.com` domain as the `data-domain` value.
+
+For more details, see the [Shopify theme docs](https://shopify.dev/docs/storefronts/themes/architecture/layouts).