refactor: KISS cleanup — deduplicate shared config, merge thin files

- Extract fonts, fish.enable, openssh.enable to modules/shared/
- Centralize sshKeys in lib/ via specialArgs
- Merge nixos/base.nix + locale.nix into nixos/default.nix
- Merge home/theme.nix into home/default.nix
- Simplify homeManagerConfig, flake packages output
- Remove redundant vim/wget from NixOS system packages
- Update AGENTS.md to reflect new structure

modules/shared/default.nix

@@ -1,3 +1,9 @@
 {
-  imports = [ ./nix.nix ];
+  imports = [
+    ./fonts.nix
+    ./nix.nix
+  ];
+
+  programs.fish.enable = true;
+  services.openssh.enable = true;
 }