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.sh

@@ -464,6 +464,13 @@ run_simple_prompts() {
   echo ""
   prompt MONITORING_NETWORK "Monitoring network name (empty = skip)" ""
 
+  echo ""
+  if prompt_yesno "Pull images from a private registry?"; then
+    prompt REGISTRY "Registry" "${REGISTRY:-$DEFAULT_REGISTRY}"
+    prompt REGISTRY_USER "Registry username" "${REGISTRY_USER:-}"
+    prompt_password REGISTRY_TOKEN "Registry token/password" "${REGISTRY_TOKEN:-}"
+  fi
+
   echo ""
   echo "  Deployment mode:"
   echo "    [1] Multi-tenant SaaS — manage platform, provision tenants on demand"