feat: update sample environment to match nixos-infra structure

- Add network/dns.nix, network/time.nix, network/proxy.nix, network/smtp.nix
- Add network/default.nix to import all network files
- Add hosts/default.nix, hosts/servers/default.nix, hosts/workstations/default.nix
- These placeholders match the structure in nixos-infra for consistency

The sample environment now reflects the same organization as production,
dev, and stage environments, making it easier to use as a reference.

environments/sample/network/default.nix

@@ -0,0 +1,16 @@
+# Central network data source for the sample environment.
+#
+# Usage from a configuration.nix:
+#   network = import ../../network { };
+#   network.dns.domain  → "sample.lagraula.fr"
+#   network.time.timeZone → "Europe/Paris"
+#
+# Usage from a shell script (via `nix eval`):
+#   nix eval --json -f network/default.nix dns
+
+{
+  dns = import ./dns.nix;
+  time = import ./time.nix;
+  proxy = import ./proxy.nix;
+  smtp = import ./smtp.nix;
+}