feat: sitemap filter excludes noindex, canonicalised and redirected URLs (tracer)

- classes.SitemapFilter implements the sitemap plugin's contract.Filter on
  the production values: redirected, noindex and canonicalised addresses
  leave the sitemaps with their reason; staging lists the same set
- the plugin requires golem15.sitemap and publishes the filter in Register
- go.mod requires sm-sitemap-plugin, go.work mounts the sibling checkout

classes/resolver.go

@@ -103,6 +103,10 @@ type MetaTag struct {
 // Resolver answers the effective SEO of a URL.
 type Resolver struct {
 	Service *Service
+	// production computes the values as production serves them: the
+	// non-production lock (D-22) is not applied. The sitemap filter decides
+	// on these, so staging lists the same addresses as production.
+	production bool
 }
 
 // Resolver returns the service's resolver.
@@ -600,7 +604,7 @@ func (r *Resolver) resolveAt(ctx context.Context, db *gorm.DB, settings models.S
 		layers = append(layers, robotsLayer{typeSetting.Robots, SourceTemplate})
 	}
 	robots, robotsSource := resolveRobots(layers)
-	if res.Staging {
+	if res.Staging && !r.production {
 		robots, robotsSource = Robots{Index: "noindex", Follow: "nofollow"}, SourceDefault
 	}
 	seo.Robots, src["robots"] = robots.Header(), robotsSource