Add script to update doomemacs to latest commit

Creates update-doomemacs.sh script that:
- Fetches latest commit SHA from doomemacs/doomemacs repo
- Automatically detects the default branch
- Updates both rev and sha256 in home/roles/emacs/default.nix
- Works from anywhere in the repo using git rev-parse

Also adds a flake app so it can be run with:
  nix run .#update-doomemacs

flake.nix

@@ -220,5 +220,21 @@
         }
       ];
     };
+
+    # Flake apps
+    apps = nixpkgs.lib.genAttrs [ "x86_64-linux" "aarch64-linux" "aarch64-darwin" ] (system:
+      let
+        pkgs = import nixpkgs { inherit system; };
+        update-doomemacs = pkgs.writeShellScriptBin "update-doomemacs" ''
+          export PATH="${pkgs.lib.makeBinPath [ pkgs.curl pkgs.jq pkgs.nix pkgs.git pkgs.gnused pkgs.gnugrep pkgs.coreutils ]}:$PATH"
+          ${builtins.readFile ./scripts/update-doomemacs.sh}
+        '';
+      in {
+        update-doomemacs = {
+          type = "app";
+          program = "${update-doomemacs}/bin/update-doomemacs";
+        };
+      }
+    );
   };
 }