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.

system/display-managers/ly.nix

@@ -0,0 +1,5 @@
+{ ... }:
+
+{
+  services.displayManager.ly.enable = true;
+}