Switch from sanboot to kernel/initrd boot with PXE overlay for ISO download

sanboot makes the ISO visible to UEFI but invisible to the Linux kernel
after ExitBootServices(). Switch to direct kernel/initrd boot with a small
CPIO overlay containing /pxe-init — a shell script that loads NIC drivers,
configures DHCP, downloads the ISO via wget, and creates a loop device
before handing off to the Proxmox installer init.

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

internal/pxe/ipxe.go

@@ -2,16 +2,26 @@ package pxe
 
 import (
 	"fmt"
+	"path/filepath"
+	"strings"
 
 	"provisioning/internal/model"
 )
 
 func BuildIPXEScript(publicURL string, img *model.Image, mac string) string {
-	isoURL := fmt.Sprintf("%s/images/boot/%s", publicURL, img.ISOPath)
+	base := fmt.Sprintf("%s/images/boot", publicURL)
+	kernelURL := fmt.Sprintf("%s/%s", base, img.KernelPath)
+	initrdURL := fmt.Sprintf("%s/%s", base, img.InitrdPath)
+	isoURL := fmt.Sprintf("%s/%s", base, img.ISOPath)
+	overlayURL := fmt.Sprintf("%s/%s/pxe-overlay.img", base,
+		strings.Split(filepath.ToSlash(img.KernelPath), "/")[0])
+	answerURL := fmt.Sprintf("%s/api/boot/auto-answer", publicURL)
 
 	return fmt.Sprintf(`#!ipxe
 echo Provisioning: booting %s on ${mac}
-echo Booting ISO...
-sanboot %s
-`, img.Name, isoURL)
+kernel %s ro ramdisk_size=16777216 rw quiet splash=silent proxmox-start-auto-installer proxmox-auto-installer-answer-url=%s prov.iso=%s rdinit=/pxe-init
+initrd %s
+initrd %s
+boot
+`, img.Name, kernelURL, answerURL, isoURL, initrdURL, overlayURL)
 }