Point the engine Explore action at the git repository

The engine pages' repo metadata (new in the wiki pages API) drives the
Explore button and card title links, with the wiki page as fallback. The
never-deployed /engines/:slug detail page and its store/api plumbing are
gone, and the engines RSS feed links to the repos too.

apps/api/app/controllers/api/wiki_controller.rb

@@ -20,6 +20,7 @@ class Api::WikiController < ApiController
       property :locale, String, desc: "Locale code"
       property :route, String, desc: "URL slug"
       property :tags, Array, of: String, desc: "Tags"
+      property :repo, String, desc: "Git repository URL (from page metadata, engines)"
       property :render, String, desc: "Rendered HTML content"
       property :content, String, desc: "Raw markdown content"
     end

apps/api/app/services/rss_service.rb

@@ -62,9 +62,8 @@ class RssService
 
       items.each do |page|
         maker.items.new_item do |item|
-          slug = page["path"].to_s.split("/").last
           item.title       = page["title"]
-          item.link        = "#{SITE_URL}/engines/#{slug}"
+          item.link        = page["repo"].presence || "#{WIKI_URL}/#{page["path"]}"
           item.description = page["description"].to_s
           item.pubDate     = Time.parse(page["createdAt"]) rescue Time.current
         end