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

extensions.go

@@ -39,6 +39,7 @@ func (p *Plugin) AdminActionExtensions() []pact.AdminActionExtension {
 	}
 	out = append(out, pact.AdminActionExtension{Controller: controllers.RulesControllerID, Form: p.ruleHistory()})
 	out = append(out, pact.AdminActionExtension{Controller: controllers.RedirectsControllerID, Form: p.redirectChecks()})
+	out = append(out, pact.AdminActionExtension{Controller: controllers.RobotsControllerID, Form: p.robotsForm()})
 	return out
 }