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>

hosts/magicman/configuration.nix

@@ -44,6 +44,18 @@
     };
   };
 
+  # Host capabilities
+  host.capabilities = {
+    hasCuda = false;
+    hasBattery = true;
+    formFactor = "laptop";
+    volumeScrollStep = 1;
+    enableArrhist = false;
+    hasEduroamAccess = true;
+    backupRepository = null;
+    backupPassword = null;
+  };
+
   # Bluetooth stuff
   services.blueman.enable = true;
   hardware.bluetooth.enable = true;