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/ssh/config.nix

@@ -22,7 +22,7 @@ in
 
     matchBlocks = {
       "bmo.uio.no-on-eduroam" = (
-        lib.mkIf (osConfig.system.name == "magicman") (
+        lib.mkIf (osConfig.host.capabilities.hasEduroamAccess) (
           lib.hm.dag.entryBefore [ "bmo.uio.no" "*" ] {
             match = "host bmo.uio.no exec \"nmcli -g GENERAL.STATE c s eduroam|grep -q -E '\\bactiv'\"";
             hostname = "bmo.uio.no";