feat: robots.txt editor and tester screen

- golem15.seo.robots singleton (UI-SPEC C10): staging callout and the
  served file (P4); the plugin's own extension adds the editor, the
  tester and the AI bots tab, saved to the robots row with history
- field type robotstxt (E4, live lint, size meter; Decode refuses over
  500 KB) and widget golem15-seo-robotstester (E5) on robots_test, which
  tests the unsaved text and switches through grobotstxt
- saving rules that block a noindex address answers robots.blocks_noindex

admin_navigation.go

@@ -38,6 +38,13 @@ func (*Plugin) Navigation() []pact.NavigationItem {
 				Permissions: []string{controllers.PermissionManage},
 				Controller:  controllers.RulesControllerID,
 			},
+			{
+				Code:        "robots",
+				Label:       "golem15.seo::lang.nav.robots",
+				Icon:        "file-text",
+				Permissions: []string{controllers.PermissionManage},
+				Controller:  controllers.RobotsControllerID,
+			},
 			{
 				Code:        "history",
 				Label:       "golem15.seo::lang.nav.history",