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

flake.nix

@@ -68,13 +68,18 @@
       };
 
       # ── Packages ────────────────────────────────────────
-      packages = nixpkgs.lib.genAttrs [ "aarch64-darwin" "x86_64-linux" ] (system: {
-        comment-checker =
-          (import nixpkgs {
+      packages = nixpkgs.lib.genAttrs [ "aarch64-darwin" "x86_64-linux" ] (
+        system:
+        let
+          pkgs = import nixpkgs {
             inherit system;
             overlays = [ self.overlays.default ];
-          }).comment-checker;
-      });
+          };
+        in
+        {
+          inherit (pkgs) comment-checker;
+        }
+      );
 
       # ── Overlays ───────────────────────────────────────
       overlays.default = import ./overlays;