Migrate nix-auto-installer into this flake

Folds the separate nix-auto-installer repo's build outputs into this
flake so it can build every auto-installer artifact it used to,
negating the need for that repo:

- modules/installer/{common,iso,proxmox-lxc}.nix — migrated from
  nix-auto-installer's common.nix/installer.nix/proxmox-lxc.nix.
- flake.nix gains nixosConfigurations.{installer,proxmox-lxc} and
  packages.x86_64-linux.{iso,lxc,pxe,all}, matching the original
  repo's interface (nix build .#iso / .#lxc / .#pxe / .#all).
- Dropped the live Gitea PAT baked into every installer image via
  environment.etc."git-credentials" — gitea.lan.ddnsgeek.com/beatzaplenty/nixos.git
  allows anonymous read, so the installer (which only ever reads the
  flake, never pushes) doesn't need a credential at all.
- installer_old.nix wasn't migrated — dead code, unreferenced by the
  source repo's flake.nix, and carried a second stale leaked token.
- The installer environment's own hardcoded login password hash is
  preserved as-is: sops-nix has no stable per-boot host key to derive
  an age key from on ephemeral installer media, so it can't reuse the
  same per-host secret mechanism the rest of this repo uses without
  separate design work.
- vars.adminSshKey / vars.dockerHost-style dedup: the installer's SSH
  authorized key and its FLAKE_BASE_URL domain were exact duplicates
  of values already in variables.nix / modules/common/configuration.nix,
  so both now reference the single source of truth instead.

Verified eval-equivalent for every existing host (drvPath-identical)
and confirmed the migrated auto-install.sh script renders byte-for-byte
identical to the source repo's output.

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

variables.nix

@@ -25,6 +25,10 @@
     "ssh-ed25519 AAAAC3NzaC1lZDI1NTE5AAAAIHxXTQxFnArK5HXG7czeoybZebCGfxpUdusJkPn+BCSp root@server"
   ];
 
+  # Admin SSH public key, authorized on the primary user of every host and
+  # the installer image's nixos/root users.
+  adminSshKey = "ssh-rsa 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 wayne@stream";
+
   # System
   timeZone = "Australia/Brisbane";
   primaryUser = "nixos"; # main interactive user on every host