Show services discovered by site agents as suggested targets

Stores the sanitised list per site, serves it at /sites/{id}/discovered and adds
it to the target picker. Needs a Reef release containing model.DiscoveredService.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

agent.go

@@ -259,6 +259,7 @@ func (s *Server) siteReport(w http.ResponseWriter, r *http.Request) {
 	answered := noteUpdateReport(&st.Version, &st.AutoUpdate, &st.UpdateID, &st.UpdateMsg, in.Version, in.AutoUpdate, in.UpdateID, in.UpdateMsg)
 	s.recordUpdateProgress(st.ID, in.UpdateID, in.UpdateStage, in.UpdatePct, in.UpdateLog, oldVersion, st.Version)
 	s.containers[st.ID] = in.Containers
+	s.discovered[st.ID] = cleanDiscovered(in.Discovered)
 	for _, x := range s.st.MailEgress {
 		if x.SiteID != st.ID {
 			continue