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/packages/default.nix

@@ -1,6 +1,6 @@
 { pkgs, osConfig, ... }:
 let
-  withCuda = if (osConfig.system.name == "gunter") then true else false;
+  withCuda = osConfig.host.capabilities.hasCuda;
 in
 {
   imports = [