refactor(hosts): rename system-manager to ubuntu

The tree is heavily Ubuntu-coupled: masking netplan's generator and Ubuntu's
own wpa_supplicant units, apt packages in cloud-init, /etc/dbus-1 policy.

Assisted-by: pi (claude-opus-5)

README.md

@@ -13,7 +13,8 @@ Looking for something simpler to start out with flakes? Try [my starter config r
 
 ```text
 hosts/nixos/    per-machine NixOS configurations (atlas, alcyone, ...)
-hosts/system-manager/ configurations for non-NixOS Linux systems
+hosts/ubuntu/   system-manager configurations for Ubuntu machines
+hosts/common/   configuration shared between both
 home/           home-manager configuration (feature-flagged)
 modules/        reusable nixos/ and home-manager/ modules
 overlays/, pkgs/ package overlays and custom packages (including wallpapers)

flake.nix

@@ -162,7 +162,7 @@
     };
 
     systemConfigs.electra = system-manager.lib.makeSystemConfig {
-      modules = [./hosts/system-manager/electra];
+      modules = [./hosts/ubuntu/electra];
       overlays = builtins.attrValues outputs.overlays;
       specialArgs = {
         inherit inputs outputs;

hosts/ubuntu/electra/cloud-init.yaml

@@ -40,7 +40,7 @@ autoinstall:
       # The installer's netplan wifi config is load-bearing for exactly one
       # boot: the bootstrap below needs the network before system-manager
       # exists to configure it. Once activated, system-manager owns
-      # /etc/systemd/network and wpa_supplicant (see hosts/system-manager/
+      # /etc/systemd/network and wpa_supplicant (see hosts/ubuntu/
       # common/optional/network.nix), and netplan's generator is masked.
       # Stop cloud-init from re-rendering network config on later boots.
       - |