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.

hosts/hivebrain/default.nix

@@ -0,0 +1,26 @@
+{ ... }:
+
+{
+  imports = [
+    ./hardware-configuration.nix
+
+    # shared
+    ../../system/boot.nix
+    ../../system/networking.nix
+    ../../system/locale.nix
+    ../../system/fonts.nix
+    ../../system/nix.nix
+    ../../system/ssh.nix
+    ../../system/programs.nix
+    ../../system/packages.nix
+
+    # marvin-only
+    ../../system/xserver.nix
+    ../../system/picom.nix
+    ../../system/display-managers/ly.nix
+    ../../system/users/marvin.nix
+  ];
+
+  networking.hostName = "hivebrain";
+  system.stateVersion = "26.05";
+}