Authorize the WSL workstation as a nix-cache remote-builder client

Adds debian@surface's existing SSH key (already used as its admin
key elsewhere in this repo) to vars.remoteBuilderAuthorizedKeys so
nix-cache will accept it as a distributed-build client once deployed.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01La55Nsss8jZ7ZuzUV9mfot

docs/pxe-boot.md

@@ -52,8 +52,15 @@ The generated menu currently exposes entries for:
 - iPXE shell
 - Reboot
 
-Kernel and initrd artifacts for the NixOS installer entry must be placed under
-`/srv/pxe/http/nixos` by an operator or a separate build process.
+The NixOS installer entry chain-loads `/srv/pxe/http/nixos/netboot.ipxe`,
+which is nixpkgs' own generated netboot iPXE script (correct `init=`/`initrd=`
+kernel parameters included) rather than a hand-rolled boot line — that script
+in turn expects its kernel/initrd siblings in the same directory. All three
+files (`bzImage`, `initrd`, `netboot.ipxe`) are built from this flake's own
+`modules/installer/iso.nix` netboot image (the same one `nix build .#pxe`
+produces) and staged automatically by
+`modules/pxe-boot/stage-installer-artifacts.nix` via `systemd.tmpfiles.rules`
+— no manual operator step required.
 
 The SystemRescue entry expects the source ISO at: