Add linting, formatting, and CI with fixes for all warnings

Infrastructure:
- Add deadnix, nixfmt, and statix to flake inputs
- Add formatter output to flake for nix fmt support
- Add deadnix, nixfmt, statix to Home Manager packages
- Add GitHub Actions workflow for CI validation
- Add .pre-commit-config.yaml with hooks for nixfmt, deadnix, and statix
- Support x86_64-darwin in formatter

Statix fixes (W10/W20 warnings):
- Remove unused lambda argument from nixpkgs-settings.nix
- Merge repeated keys in hardware-configuration.nix files (boot.initrd, boot, fileSystems)
- Merge repeated keys in nixnuc/default.nix (services, virtualisation)
- Merge repeated keys in rainbow-planet/default.nix (desktopManager)
- Merge repeated keys in home/general/default.nix (home)

Deadnix fixes (unused declarations):
- Remove unused pkgs/lib/username/http_port arguments from various files
- Fix unused final parameter in overlay functions (final -> _final)

CI/pre-commit fixes:
- Fix pre-commit statix config: add pass_filenames: false
- Fix CI workflow: use nix run nixpkgs# prefix and --ci flag for nixfmt

examples/flake-structure.nix

@@ -1,49 +1,46 @@
 {
-  inputs = {};
-  outputs = inputs@{}: let
-    darwinHostConfig = { system, hostname, username, additionalModules, additionalSpecialArgs }:
-      nix-darwin.lib.darwinSystem { };
-
-    mkNixosHost = { system, hostname, username, additionalModules, additionalSpecialArgs }:
-      nixpkgs.lib.nixosSystem { };
-    
-    linuxHomeConfig = { system, hostname, username, additionalModules, additionalSpecialArgs }:
-      home-manager.lib.homeManagerConfiguration { };
-
-  in {
-    # Darwin (macOS) hosts
-    darwinConfigurations = {
-      mightymac = darwinHostConfig {
-        system = "aarch64-darwin";
-        hostname = "mightymac";
-        username = "gene.liverman";
-        additionalModules = [];
-        additionalSpecialArgs = {};
-      };
-    };
-
-    # NixOS hosts
-    nixosConfigurations = {
-      rainbow-planet = mkNixosHost {
-        system = "x86_64-linux";
-        hostname = "rainbow-planet";
-        username = "gene";
-        additionalModules = [
-          nixos-hardware.nixosModules.dell-xps-13-9360
-        ];
-        additionalSpecialArgs = {};
-      };
-    };
-
-    # Home Manager (only) users
-    homeConfigurations = {
-      gene = linuxHomeConfig {
-        system = "x86_64-linux";
-        hostname = "mini-watcher";
-        username = "gene";
-        additionalModules = [];
-        additionalSpecialArgs = {};
-      };
-    };
+  inputs = {
+    nixpkgs.url = "github:nixos/nixpkgs/nixos-25.05";
+    home-manager.url = "github:nix-community/home-manager";
+    nix-darwin.url = "github:lnl7/nix-darwin";
+    nixos-hardware.url = "github:NixOS/nixos-hardware";
   };
-}
+
+  outputs =
+    inputs@{ self, ... }:
+    let
+      # Import helper functions from lib/
+      localLib = import ./lib { inherit inputs; };
+    in
+    {
+      # Darwin (macOS) hosts
+      darwinConfigurations = {
+        mightymac = localLib.mkDarwinHost {
+          system = "aarch64-darwin";
+          hostname = "mightymac";
+          username = "gene.liverman";
+        };
+      };
+
+      # NixOS hosts
+      nixosConfigurations = {
+        rainbow-planet = localLib.mkNixosHost {
+          system = "x86_64-linux";
+          hostname = "rainbow-planet";
+          username = "gene";
+          additionalModules = [
+            inputs.nixos-hardware.nixosModules.dell-xps-13-9360
+          ];
+        };
+      };
+
+      # Home Manager (only) users
+      homeConfigurations = {
+        gene = localLib.mkHomeConfig {
+          system = "x86_64-linux";
+          homeDirectory = "/home/gene";
+          username = "gene";
+        };
+      };
+    };
+}