Add devShell with agenix and colmena

.envrc

@@ -0,0 +1,2 @@
+# shellcheck shell=bash
+use flake

.gitignore

@@ -1,3 +1,4 @@
 result
 .gcroots
 config.tf.json
+.direnv

flake.nix

@@ -57,7 +57,15 @@
           '');
         };
     };
-    defaultApp.${system} = self.apps.${system}.apply;
+    apps.${system}.default = self.apps.${system}.apply;
+
+    devShells.${system}.default = pkgs.mkShell {
+      packages = [ 
+        inputs.agenix.packages.${system}.agenix
+        inputs.colmena.packages.${system}.colmena
+      ];
+    };
+
     colmena = {
       meta.nixpkgs = import nixpkgs {
         localSystem = system;