Fix auto-install.sh to work standalone, not just baked into the image

Two real bugs, both hit live:

1. Shebang: #!/run/current-system/sw/bin/bash only resolves on an
   already-activated NixOS system -- running the checked-out script
   directly (e.g. from a stock ISO, cloned repo) failed with "cannot
   execute: required file not found" on a non-NixOS box. Switched to
   #!/usr/bin/env bash, which resolves identically on NixOS
   (environment.usrbinenv's own default) and any normal Linux distro.
   Also fixed the file's missing executable bit.

2. FLAKE_BASE_URL: previously depended on pkgs.replaceVars substituting
   a Nix-templated @lanDomain@ placeholder at build time -- meaning it
   only ever worked when baked into the built installer image, not when
   run straight from a checkout (the literal, unexpanded "@lanDomain@"
   string reached git as a bogus hostname). Replaced with LAN_DOMAIN in
   scripts/env.sh (manually kept in sync with variables.nix's lanDomain,
   same pattern as NIX_CACHE_HOST/nixCacheHost already), sourced by the
   script itself like every other script in scripts/. Dropped
   pkgs.replaceVars from modules/installer/common.nix entirely --
   scripts/env.sh is now baked into the image alongside auto-install.sh
   at a matching relative path (/etc/nixos-installer/env.sh next to
   /etc/nixos-installer/installer/auto-install.sh) so the script's own
   relative `source` line resolves the same way in both contexts.

loginShellInit's invocation path and docs/auto-installer.md updated to
match. Verified: shellcheck clean on both scripts, the baked files are
byte-identical to their checked-in sources (no templating left to
verify), and codex-maintenance.sh (secret grep, fmt, statix, full eval
of every host/package including the installer/pxe artifacts) passes
clean.

docs/auto-installer.md

@@ -8,10 +8,13 @@ lives here.
 The installer provides a small NixOS install environment (ISO, or the same
 image netbooted via PXE) with SSH access, Git support, and an interactive
 installation script.
-Logging in as any user (root or `nixos`) runs `/etc/auto-install.sh`,
-discovers available hosts from this same flake, lets the operator choose a
-target, applies that host's Disko storage configuration, installs NixOS, and
-reboots.
+Logging in as any user (root or `nixos`) runs
+`/etc/nixos-installer/installer/auto-install.sh` (the same file as
+`scripts/installer/auto-install.sh` in this repo — see "Installer process"
+below for why it's baked in at that path rather than a flat
+`/etc/auto-install.sh`), discovers available hosts from this same flake,
+lets the operator choose a target, applies that host's Disko storage
+configuration, installs NixOS, and reboots.
 
 **This applies to every `nixosConfigurations` target except `lxc-*` hosts —
 see "LXC hosts" immediately below for why those are different.**
@@ -217,7 +220,21 @@ entirely (see "LXC hosts" above), so it never reaches this code path.
 
 ## Installer process
 
-`/etc/auto-install.sh`:
+`scripts/installer/auto-install.sh` is a real, version-controlled shell
+script — not an inline Nix string. It sources `scripts/env.sh` for
+`LAN_DOMAIN` itself (same as every other script in `scripts/`), so it
+behaves identically whether it's run straight from a git checkout (e.g.
+manually, from a stock NixOS ISO that isn't this repo's own installer
+image) or from inside the built installer image. That's also why it's
+baked in at `/etc/nixos-installer/installer/auto-install.sh` rather than a
+flat `/etc/auto-install.sh` — `modules/installer/common.nix` bakes
+`scripts/env.sh` in alongside it at `/etc/nixos-installer/env.sh`,
+preserving the same relative layout (`installer/auto-install.sh` ->
+`../env.sh`) the checked-out repo has, so the script's own
+`source ".../env.sh"` line resolves correctly in both places without any
+Nix-level templating.
+
+Once running, it:
 
 1. Queries `nixosConfigurations` from this flake over the network (`git+https://<lanDomain>/beatzaplenty/nixos.git`) — this happens at *install* time, not build time, so a generic installer image always sees whatever hosts are currently committed, without needing a rebuild.
 2. Presents them as a menu; confirms the choice.