Build the Skeleton and a minimal neogaia Host (task 0001)

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  and hardware profile, all superseded by disko in task 0002.
- CLAUDE.md: project agent instructions with a Gotchas section (nix on the
  CachyOS dev host, the chaotic overlay/cache behaviour, tea/Gitea).

flake.nix

@@ -0,0 +1,42 @@
+{
+  description = "Alexion's NixOS configuration — one flake for every Host";
+
+  inputs = {
+    # Base channel: nixos-unstable (rolling, but gated by the NixOS test suite).
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-unstable";
+
+    # Fresher-than-base packages, reachable per-package as `unstable.<name>`.
+    nixpkgs-unstable.url = "github:nixos/nixpkgs/nixpkgs-unstable";
+
+    # Latest stable release, reachable per-package as `stable.<name>`.
+    nixpkgs-stable.url = "github:nixos/nixpkgs/nixos-25.05";
+
+    home-manager = {
+      url = "github:nix-community/home-manager";
+      inputs.nixpkgs.follows = "nixpkgs";
+    };
+
+    # CachyOS kernel + binary cache. Deliberately NOT following our nixpkgs, so the
+    # chaotic cache stays usable and the kernel is fetched rather than compiled.
+    chaotic.url = "github:chaotic-cx/nyx/nyxpkgs-unstable";
+  };
+
+  outputs =
+    { self, nixpkgs, ... }@inputs:
+    let
+      inherit (nixpkgs) lib;
+      my = import ./lib { inherit lib inputs self; };
+    in
+    {
+      # The trimmed helper lib: the Auto-loader, the host-builder, the script-from-file helper.
+      lib = my;
+
+      # Every Host under hosts/ is auto-discovered and built.
+      nixosConfigurations = my.mkHosts (self + "/hosts");
+
+      # `nix flake check` builds each Host's toplevel — the primary test seam.
+      checks.x86_64-linux = lib.mapAttrs (
+        _name: host: host.config.system.build.toplevel
+      ) self.nixosConfigurations;
+    };
+}