2.0.1: Register sitemap provider with key/label so admin can toggle it

Updates the seo_sitemap_providers registration to the new
{key, label, callback} shape (s-cart ADR seo_plugin-sitemap-extension,
amended 20260711T135121) so the "SEO" admin screen can list and
toggle this plugin's sitemap contribution independently. Reuses the
manifest's own configKey/name fields — no new translation string
needed just for this.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>