feat(installer): add interactive registry prompts

Both simple and expert modes now ask "Pull images from a private
registry?" with follow-up prompts for URL, username, and token.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

installer/install.ps1

@@ -484,7 +484,14 @@ function Run-SimplePrompts {
  
     Write-Host ''
     $c.MonitoringNetwork = Prompt-Value 'Monitoring network name (empty = skip)' ''
- 
+
+    Write-Host ''
+    if (Prompt-YesNo 'Pull images from a private registry?') {
+        $c.Registry      = Prompt-Value 'Registry' (Coalesce $c.Registry $DEFAULT_REGISTRY)
+        $c.RegistryUser  = Prompt-Value 'Registry username' (Coalesce $c.RegistryUser '')
+        $c.RegistryToken = Prompt-Password 'Registry token/password' (Coalesce $c.RegistryToken '')
+    }
+
     Write-Host ''
     Write-Host '  Deployment mode:'
     Write-Host '    [1] Multi-tenant SaaS -- manage platform, provision tenants on demand'