Restructure flake into per-host modules

Move host-specific files under hosts/<name>/ so a second machine can be
added without forking the repo. Split system/users.nix and
system/display-manager.nix into per-option subfolders, separate the
home-manager packages list into a shared base and a personal bundle,
extract networking.hostName out of the shared networking module, and
inline the git identity into the per-host home.nix. Add .gitignore
guardrails to keep secrets out.

flake.nix

@@ -9,20 +9,27 @@
 		};
 	};
 
-	outputs = { self, nixpkgs, home-manager, ... }: {
-		nixosConfigurations.hivebrain = nixpkgs.lib.nixosSystem {
+	outputs = { self, nixpkgs, home-manager, ... }:
+	let
+		mkHost = { hostPath, username, homePath }: nixpkgs.lib.nixosSystem {
 			system = "x86_64-linux";
 			modules = [
-				./configuration.nix
+				hostPath
 				home-manager.nixosModules.home-manager {
 					home-manager = {
 						useGlobalPkgs = true;
 						useUserPackages = true;
-						users.marvin = import ./home.nix;
+						users.${username} = import homePath;
 						backupFileExtension = "backup";
 					};
 				}
 			];
 		};
+	in {
+		nixosConfigurations.hivebrain = mkHost {
+			hostPath = ./hosts/hivebrain;
+			username = "marvin";
+			homePath = ./hosts/hivebrain/home.nix;
+		};
 	};
 }