Rename PXE installer menu entry, add vanilla NixOS minimal netboot entry

The iPXE menu's "nixos" entry actually chain-loads this flake's own
custom auto-installer image, not a stock NixOS image — rename it to
"auto-installer" (label "NixOS Auto-Installer") so the menu says what it
boots, and set networking.hostName on netbootSystem to match, so the
generated system name (nixos-system-auto-installer-*) and staged
directory (/srv/pxe/http/auto-installer) agree with the menu entry too.

Add a second, genuinely vanilla NixOS minimal netboot image
(netbootMinimalSystem in flake.nix — nixpkgs' netboot-minimal.nix on its
own, none of modules/installer/common.nix's auto-installer wiring),
built from source the same way as the auto-installer image and exposed
as packages.x86_64-linux.pxe-minimal. Staged and menu-wired the same
way, as "nixos-minimal" (item, hostname, and directory all matching).

modules/pxe-boot/stage-installer-artifacts.nix is generalized to stage
both images via a shared rule-builder instead of one hardcoded set of
paths.

Verified: nix eval confirms both images' config.system.name matches
their menu entry/directory names, the pxe-boot host itself builds
clean with the new menu.ipxe, and the new pxe-minimal image was booted
directly under QEMU (kernel+initrd, no KVM) to a working login shell
with hostname nixos-minimal, no hang.

docs/auto-installer.md

@@ -150,7 +150,11 @@ use case.
 
 The `pxe` variant is also built automatically as part of the `pxe-boot` host
 itself (`modules/pxe-boot/stage-installer-artifacts.nix`) and served over
-iPXE — see `docs/pxe-boot.md`.
+iPXE as the menu's "NixOS Auto-Installer" entry — see `docs/pxe-boot.md`.
+That same host also builds and serves `packages.x86_64-linux.pxe-minimal`,
+a vanilla NixOS minimal netboot image with none of this auto-installer's
+wiring, as a separate "NixOS Minimal" menu entry — also documented in
+`docs/pxe-boot.md`, not covered further here since it's not this installer.
 
 ## Host keys