seo(storefront): exclude old-slug redirect URLs from sitemap.xml#292

What happened

The generated sitemap-0.xml lists 581 /products/ URLs, but 185 of them are old-slug pages that 301 to their canonical URL. Search engines get a sitemap full of redirects.

The [slug].astro getStaticPaths() emits both canonical and redirect paths; the sitemap integration filter does not distinguish them.

Plan

Filter redirect entries out of the sitemap (the integration filter receives the page URL; exclude slugs that are not canonical, or pass canonical-only data to the sitemap integration).

Acceptance criteria

• Sitemap contains exactly the 330 canonical product URLs plus non-product public pages
• Zero redirect URLs in any sitemap file