feat(#2228): add XDG icon set and .desktop entries for hyperhive processes

Adds hive-xdg-icons package (nix/packages/hive-xdg-icons.nix) that
rasterizes the branding SVG to all standard hicolor sizes (16–256px)
and installs /usr/share/pixmaps/hyperhive.png as a flat fallback.
Ships .desktop entries for hive-c0re and hive-ag3nt with NoDisplay=true
so desktop environments can match running processes to their icon without
cluttering the application launcher.

Narrow drv input: only ./branding/hyperhive.svg, so unrelated source
changes don't bust the cache.

Wired into environment.systemPackages in hive-c0re.nix (host side)
so the icons are present wherever hive-c0re is deployed.

Closes #2228

nix/modules/hive-c0re.nix

@@ -4,6 +4,7 @@
   hyperhiveAssets,
   hyperhiveFlake,
   hyperhiveDocs,
+  hyperhiveXdgIcons,
   agentBaseToplevel,
   managerToplevel,
 }:
@@ -794,6 +795,9 @@ in
     environment.systemPackages = [
       cfg.package
       pkgs.git
+      # XDG icons + .desktop entries so desktop environments can match
+      # hyperhive processes to their icon (task managers, CPU monitors, etc.).
+      (hyperhiveXdgIcons pkgs.stdenv.hostPlatform.system)
     ];
 
     # Serve config at a stable /etc path so hive-c0re's ExecStart