refactor: Move network configurations to machine directories

- Move network-congenital-optimist.nix to machines/congenital-optimist/
- Move network-sleeper-service.nix to machines/sleeper-service/
- Update import paths in machine configurations
- Clean up modules/network/common.nix to remove SSH duplication
- Consolidate SSH configuration in modules/security/ssh-keys.nix
- Remove machine-specific networking from shared common module

This improves dependency tracking by co-locating machine-specific
network configurations with their respective machines.

modules/security/ssh-keys.nix

@@ -3,6 +3,9 @@
 { config, pkgs, lib, ... }:
 
 {
+  # Firewall configuration for SSH
+  networking.firewall.allowedTCPPorts = [ 22 ];
+
   # Global SSH daemon configuration
   services.openssh = {
     enable = true;