Install OpenSSH Server during Windows setup

Install it at first logon from the Win32-OpenSSH zip on the seed ISO via
Configure-Windows.ps1 -InstallOpenSSH, replacing the manual 05/06 batch
files. The administrator public key now comes from the host
(SSH_PUBLIC_KEY_FILE or ~/.ssh/id_*.pub) instead of being hardcoded.

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

README.md

@@ -174,14 +174,13 @@ VirtIO ISO with `pnputil` on arm64), without which the guest has no network on
 arm64. The installer includes the VirtIO drivers, SPICE vdagent, and QEMU Guest
 Agent. The launcher exposes the vdagent channel for host/guest clipboard
 integration and the QEMU Guest Agent at `.vm/windows11/qemu-guest-agent.sock`.
-Change the password after first login. The OpenSSH Server is not installed
-automatically. Run `05-Install-OpenSSH-Admin.bat` from the `Installers` folder on
-the desktop to install it through Windows Update, which can be slow, or
-`06-Install-OpenSSH-Zip-Admin.bat` to install it from the Win32-OpenSSH zip for
-the host architecture, which the seed ISO carries (downloaded by
-`vm-images/download-images.sh openssh-win`), as
-administrator, then connect with `ssh guofang@localhost -p 2222`; it also installs
-the host's configured SSH public key for passwordless administrator login.
+Change the password after first login. The OpenSSH Server is installed during
+the first logon from the Win32-OpenSSH zip for the host architecture, which the
+seed ISO carries (downloaded by `vm-images/download-images.sh openssh-win`), so
+Windows Update is not needed. Connect with `ssh guofang@localhost -p 2222`; the
+seed ISO also carries the host's SSH public key (`$SSH_PUBLIC_KEY_FILE`, else the
+first of `~/.ssh/id_ed25519.pub`, `id_ecdsa.pub`, `id_rsa.pub`) for passwordless
+administrator login.
 The Windows VM hardware (8G memory, 8 CPUs, system disk, devices)
 is defined in `qemu/windows11.ini`, with the accelerators and machine type in
 `qemu/windows11-<os>-<arch>.ini`; the launcher renders the former to