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

plugin.go

@@ -14,6 +14,7 @@ import (
 	"git.golem15.com/golem15/sm-seo-plugin/classes"
 	"git.golem15.com/golem15/sm-seo-plugin/models"
 	"git.golem15.com/golem15/sm-seo-plugin/updates"
+	"git.golem15.com/golem15/sm-sitemap-plugin/contract"
 	"git.golem15.com/golem15/summercms/modules/backpack"
 	"git.golem15.com/golem15/summercms/modules/pact"
 	"git.golem15.com/golem15/summercms/modules/party"
@@ -46,13 +47,21 @@ type Plugin struct {
 
 func (p *Plugin) ID() string { return "golem15.seo" }
 
-// Requires is empty: content types are found through signpost, never by
-// plugin ID.
-func (p *Plugin) Requires() []string { return nil }
+// SitemapPluginID is the sitemap plugin the SEO plugin requires: it
+// decides which addresses the sitemaps list (D-95). Content types are still
+// found through signpost, never by plugin ID.
+const SitemapPluginID = "golem15.sitemap"
 
+// Requires names the sitemap plugin, whose Filter seam the SEO plugin
+// implements (D-95).
+func (p *Plugin) Requires() []string { return []string{SitemapPluginID} }
+
+// Register publishes the sitemap filter (classes.SitemapFilter): noindex,
+// canonicalised and redirected addresses leave the sitemaps (D-95, D-24).
+// It reads the service lazily, since Boot builds the service.
 func (p *Plugin) Register(app *backpack.App) error {
 	p.app = app
-	return nil
+	return app.Publish[contract.Filter](classes.SitemapFilter{Service: p.Service})
 }
 
 // Boot builds the service on the application's database, signpost registry