system: replace host detection with capabilities module

Replace scattered osConfig.system.name comparisons with a declarative
host.capabilities module. This improves maintainability and semantic
clarity by expressing what capabilities a host has rather than checking
its name.

Changes:
- Add system/host-capabilities.nix with options for hardware, form factor,
  UI behavior, services, and backup configuration
- Configure capabilities in hosts/gunter and hosts/magicman
- Migrate 6 files to use capabilities: packages, waybar, ssh, backup-home
- Remove redundant host name check for pciutils in gunter config
- Make backup-home service conditionally enabled based on capabilities

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

home/hyprland/waybar/default.nix

@@ -14,8 +14,9 @@ let
     dontUnpack = true;
     installPhase = "install -Dm755 ${./arrhist.py} $out/bin/arrhist";
   };
-  withArrhist = if (osConfig.system.name == "gunter") then true else false;
-  withBattery = if (osConfig.system.name == "magicman") then true else false;
+  cfg = osConfig.host.capabilities;
+  withArrhist = cfg.enableArrhist;
+  withBattery = cfg.hasBattery;
 in
 {
   sops.secrets."sonarr_base_url" = { };
@@ -153,7 +154,7 @@ in
     settings = [
       (
         let
-          volInterval = if (osConfig.system.name == "gunter") then "5" else "1";
+          volInterval = toString cfg.volumeScrollStep;
         in
         {
           "layer" = "top";