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
