Add an openssh-win target to download the Win32-OpenSSH zip

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

README.md

@@ -51,7 +51,8 @@ ISO (for example `Win11_25H2_English_x64_v2.iso` →
 existing `*Win*11*x64*.qcow2` (`*Win*11*arm64*.qcow2` on arm64) in `base-qcow2/` is used instead.
 
 Download URLs can be overridden with `DEBIAN_IMAGE_URL`, `ALMA_IMAGE_URL`,
-`FEDORA_IMAGE_URL`, `UBUNTU_IMAGE_URL`, and `VIRTIO_WIN_ISO_URL`. Each may
+`FEDORA_IMAGE_URL`, `UBUNTU_IMAGE_URL`, `VIRTIO_WIN_ISO_URL`, and
+`OPENSSH_WIN_ZIP_URL`. Each may
 list several mirrors of the same file, separated by whitespace; the Fedora
 image defaults to the TUNA, BFSU and NJU mirrors, and the Ubuntu image to the
 TUNA, NJU and official cloud-images.ubuntu.com mirrors.
@@ -63,6 +64,10 @@ download is only resumed with the same downloader and mirror list; otherwise
 it starts over. The Fedora and Ubuntu checksums can be overridden with
 `FEDORA_IMAGE_SHA256` and `UBUNTU_IMAGE_SHA256` when using a custom image.
 
+The `openssh-win` target downloads the Win32-OpenSSH zip matching the
+architecture into `tools-iso/`: `OpenSSH-Win64.zip` on amd64 hosts and
+`OpenSSH-ARM64.zip` on arm64 hosts.
+
 ## Windows installation disk image
 
 QEMU's emulated USB CD-ROM reads the Windows ISO at only a few MB/s on macOS.