ui: show no-agents banner on dashboard with link to onboarding step 2

When a company has no agents (e.g. user exited onboarding early), the
dashboard now shows an amber banner: "You have no agents. Create one
here" that opens the onboarding wizard directly at step 2 (agent
creation), skipping the company creation step.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

ui/src/pages/Companies.tsx

@@ -90,7 +90,7 @@ export function Companies() {
   return (
     <div className="space-y-6">
       <div className="flex items-center justify-end">
-        <Button size="sm" onClick={openOnboarding}>
+        <Button size="sm" onClick={() => openOnboarding()}>
           <Plus className="h-3.5 w-3.5 mr-1.5" />
           New Company
         </Button>