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.

home/packages-base.nix

@@ -0,0 +1,10 @@
+{ pkgs, ... }:
+
+{
+  home.packages = with pkgs; [
+    neovim
+    nerd-fonts.jetbrains-mono
+    ripgrep
+    unzip
+  ];
+}