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
 

docs/pxe-boot.md

@@ -1,9 +1,10 @@
 # pxe-boot
 
-The `pxe-boot` host serves HTTP boot assets for iPXE clients — including a
-self-staged copy of this flake's own auto-installer netboot image, see
-`docs/auto-installer.md` for what that image actually is and does once
-booted.
+The `pxe-boot` host serves HTTP boot assets for iPXE clients — including
+self-staged copies of both this flake's own auto-installer netboot image
+(see `docs/auto-installer.md` for what that image actually is and does once
+booted) and a vanilla, unmodified NixOS minimal netboot image for plain
+rescue/inspection use.
 
 ## Host Role
 
@@ -28,7 +29,8 @@ The host creates these directories with systemd tmpfiles:
 /srv/pxe
 /srv/pxe/http
 /srv/pxe/http/images
-/srv/pxe/http/nixos
+/srv/pxe/http/auto-installer
+/srv/pxe/http/nixos-minimal
 /srv/pxe/http/systemrescue
 /srv/pxe/http/ubuntu
 /srv/pxe/http/rescue
@@ -37,7 +39,7 @@ The host creates these directories with systemd tmpfiles:
 
 Mount shared image storage under `/srv/pxe/http`, preferably
 `/srv/pxe/http/images` unless a menu entry expects files in a specific
-directory such as `/srv/pxe/http/nixos`.
+directory such as `/srv/pxe/http/auto-installer`.
 
 The HTTP iPXE chain is:
 
@@ -50,20 +52,37 @@ undionly.kpxe or ipxe.efi
 
 The generated menu currently exposes entries for:
 
-- NixOS installer
+- NixOS Auto-Installer
+- NixOS Minimal
 - SystemRescue environment
 - iPXE shell
 - Reboot
 
-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.
+Both NixOS entries chain-load a `netboot.ipxe` staged into their own
+directory (`/srv/pxe/http/auto-installer/netboot.ipxe` and
+`/srv/pxe/http/nixos-minimal/netboot.ipxe`), each 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. Each directory's three files
+(`bzImage`, `initrd`, `netboot.ipxe`) are built from source and staged
+automatically by `modules/pxe-boot/stage-installer-artifacts.nix` via
+`systemd.tmpfiles.rules` — no manual operator step required:
+
+- `auto-installer` is this flake's own `netbootSystem` (`flake.nix`) — the
+  same auto-installer image `nix build .#pxe` produces. See
+  `docs/auto-installer.md`.
+- `nixos-minimal` is `netbootMinimalSystem` (`flake.nix`) — nixpkgs'
+  `netboot-minimal.nix` composed on its own, with none of this flake's
+  auto-installer wiring (no `common.nix`, no `auto-install.sh`, no baked
+  host keys or custom users). Same `nix build .#pxe-minimal` mechanism as
+  the auto-installer image, just a different module composition. Useful
+  as a plain rescue/inspection shell that doesn't assume anything about
+  this flake.
+
+Both images set `networking.hostName` to match their menu entry/staged
+directory name (`auto-installer` / `nixos-minimal`), so each one's
+generated system name (`nixos-system-<name>-*`) is self-describing rather
+than the nixpkgs default of `nixos-system-nixos-*` for both.
 
 The SystemRescue entry expects the source ISO at: