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/hosts/servers/default.nix

@@ -0,0 +1,15 @@
+{ config, pkgs, lib, ... }:
+
+{
+  # Common settings for all servers in the sample environment
+  # This file is imported by all server configurations
+
+  # TODO: Move common server settings here later
+  # Examples:
+  # - Server-specific users/groups
+  # - Server-specific packages
+  # - Server-specific services
+  # - Server-specific security policies
+  # - Server-specific monitoring/alerting
+  # - Locale: en_US.UTF-8 (for servers)
+}