feat: add dev and stage environment skeletons

- Create environments/dev/ and environments/stage/ with configuration.nix,
  network/, secrets/, and hosts/ directories
- Both environments pin to the same framework commit as production
- Dev and stage subdomains (dev.lagraula.fr, stage.lagraula.fr) ready for
  DNS server configurations when containers are deployed

nixos-infra/environments/dev/configuration.nix

@@ -0,0 +1,19 @@
+{ config, pkgs, lib, ... }:
+
+let
+  # Pin to the same framework commit as production — update when testing
+  # new framework features in dev before rolling to production.
+  frameworkRev = "c53d997d075236f6d8c2a8e9db0238e46391735a";
+  framework = builtins.fetchGit {
+    url = "ssh://git@gitea.prod.lagraula.fr:2222/xavier/nixos-infra-framework.git";
+    rev = frameworkRev;
+  };
+in {
+  imports = [
+    (framework + "/lib")
+    (framework + "/overlays/custom-pkgs.nix")
+  ];
+
+  # Dev environment-wide settings
+  system.stateVersion = "25.11";
+}

nixos-infra/environments/stage/configuration.nix

@@ -0,0 +1,19 @@
+{ config, pkgs, lib, ... }:
+
+let
+  # Pin to the same framework commit as production — update when testing
+  # new framework features in staging before rolling to production.
+  frameworkRev = "c53d997d075236f6d8c2a8e9db0238e46391735a";
+  framework = builtins.fetchGit {
+    url = "ssh://git@gitea.prod.lagraula.fr:2222/xavier/nixos-infra-framework.git";
+    rev = frameworkRev;
+  };
+in {
+  imports = [
+    (framework + "/lib")
+    (framework + "/overlays/custom-pkgs.nix")
+  ];
+
+  # Staging environment-wide settings
+  system.stateVersion = "25.11";
+}