Build the Skeleton and a minimal neogaia Host

Stand up the walking skeleton the rest of the laptop MVI extends and
re-verifies against: the whole neogaia Host evaluates and its system
toplevel builds (nix flake check green).

- flake.nix: hand-rolled flake (no flake-parts). Base nixos-unstable, plus
  nixpkgs-unstable and nixos-25.05 for the per-package unstable/stable
  overlays, home-manager (nixpkgs followed), and chaotic-nyx (deliberately
  not following our nixpkgs, to keep its binary cache usable). checks build
  each Host toplevel.
- lib/: trimmed helper lib — the Auto-loader (recursive .nix discovery, no
  null-placeholder hack), the host-builder, and the script-from-file helper.
  Deps inherited explicitly; no with lib.my, no nixosModules output.
- system/: shared base config — the unstable/stable overlays, the user
  option (defaults to alexion, in wheel, drives system + home-manager user
  in lockstep), flakes, git, and home-manager as a NixOS module.
- modules/example.nix: Auto-loader / Enable-convention reference Module,
  inert until enabled.
- hosts/neogaia/: minimal laptop Host — placeholder filesystems, bootloader,
  and hardware profile.
- CLAUDE.md: project agent instructions with a Gotchas section (nix on the
  CachyOS dev host, the chaotic overlay/cache behaviour, the forge CLI).

lib/default.nix

@@ -0,0 +1,83 @@
+{
+  lib,
+  inputs,
+  self,
+}:
+let
+  inherit (lib)
+    attrNames
+    filterAttrs
+    genAttrs
+    flatten
+    hasSuffix
+    mapAttrsToList
+    ;
+
+  # --- Auto-loader ---------------------------------------------------------
+  # Recursively collect every `.nix` file under `dir`, returned as a flat list
+  # of paths suitable for a module `imports`. No null-placeholder traversal
+  # hack: a directory recurses, a `.nix` file is taken, anything else is skipped.
+  collectNixFiles =
+    dir:
+    flatten (
+      mapAttrsToList (
+        name: type:
+        let
+          path = dir + "/${name}";
+        in
+        if type == "directory" then
+          collectNixFiles path
+        else if type == "regular" && hasSuffix ".nix" name then
+          [ path ]
+        else
+          [ ]
+      ) (builtins.readDir dir)
+    );
+
+  # --- Host-builder --------------------------------------------------------
+  # Build one Host: every Module is imported unconditionally (inert until its
+  # `enable` flag is set), alongside home-manager, chaotic, the shared base,
+  # and the Host's own directory.
+  mkHost =
+    {
+      hostName,
+      system ? "x86_64-linux",
+    }:
+    inputs.nixpkgs.lib.nixosSystem {
+      inherit system;
+      specialArgs = {
+        inherit inputs;
+        my = self.lib;
+      };
+      modules =
+        (collectNixFiles (self + "/modules"))
+        ++ [
+          inputs.home-manager.nixosModules.home-manager
+          inputs.chaotic.nixosModules.default
+          (self + "/system")
+          (self + "/hosts/${hostName}")
+          { networking.hostName = hostName; }
+        ];
+    };
+
+  # Discover every Host (a subdirectory of `hostsDir`) and build each one.
+  mkHosts =
+    hostsDir:
+    let
+      hostNames = attrNames (filterAttrs (_name: type: type == "directory") (builtins.readDir hostsDir));
+    in
+    genAttrs hostNames (hostName: mkHost { inherit hostName; });
+
+  # --- Script-from-file helper --------------------------------------------
+  # Turn a standalone script file into a package on PATH, keeping the script
+  # itself editable as a real file rather than an inlined heredoc.
+  scriptFromFile = pkgs: name: path: pkgs.writeShellScriptBin name (builtins.readFile path);
+in
+{
+  inherit
+    collectNixFiles
+    mkHost
+    mkHosts
+    scriptFromFile
+    ;
+}