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

flake.nix

@@ -236,6 +236,12 @@
           # nix/reference-docs.nix. (`docs` above is the auto-generated
           # nix-options reference, a different artifact.)
           reference-docs = pkgs.callPackage ./nix/reference-docs.nix { };
+          # XDG icon set + .desktop entries for hyperhive processes.
+          # Narrow input: only the branding SVG, so unrelated source changes
+          # don't bust this derivation's cache.
+          xdg-icons = pkgs.callPackage ./nix/packages/hive-xdg-icons.nix {
+            hyperhiveSvg = ./branding/hyperhive.svg;
+          };
           # Pre-built per-container system closures. Exposed as packages
           # so operators can `nix build .#agent-base-toplevel` (or wire
           # them into their host system closure via the
@@ -298,6 +304,7 @@
           hyperhivePackage = system: self.packages.${system}.default;
           hyperhiveFrontend = system: self.packages.${system}.frontend;
           hyperhiveAssets = system: self.packages.${system}.assets;
+          hyperhiveXdgIcons = system: self.packages.${system}.xdg-icons;
           hyperhiveFlake = "${hyperhiveFlakeSource}";
           # Narrow docs/ source, threaded as its own meta-flake input so
           # doc edits don't re-hash the whole flake source.

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

nix/packages/hive-xdg-icons.nix

@@ -0,0 +1,85 @@
+{
+  stdenv,
+  librsvg,
+  # Absolute path to the hyperhive.svg source so this derivation's input
+  # hash is narrow: a change to any other source file does NOT bust it.
+  hyperhiveSvg,
+}:
+
+# XDG icon set + .desktop entries for hyperhive processes so desktop
+# environments can match running processes to icons (task managers, CPU
+# monitors, etc.).
+#
+# Output layout:
+#   $out/share/icons/hicolor/<N>x<N>/apps/hyperhive.png  (N = 16,32,48,64,128,256)
+#   $out/share/pixmaps/hyperhive.png                     (48px fallback)
+#   $out/share/applications/hive-c0re.desktop
+#   $out/share/applications/hive-ag3nt.desktop
+stdenv.mkDerivation {
+  pname = "hive-xdg-icons";
+  version = "0.1.0";
+
+  # No source tree — everything is built from the single SVG path.
+  dontUnpack = true;
+
+  nativeBuildInputs = [ librsvg ];
+
+  buildPhase = ''
+    runHook preBuild
+    mkdir -p icons
+    for size in 16 32 48 64 128 256; do
+      rsvg-convert --width "$size" --height "$size" \
+        -o "icons/hyperhive-''${size}.png" \
+        ${hyperhiveSvg}
+    done
+    runHook postBuild
+  '';
+
+  installPhase = ''
+    runHook preInstall
+    # Per-size hicolor tree
+    for size in 16 32 48 64 128 256; do
+      mkdir -p "$out/share/icons/hicolor/''${size}x''${size}/apps"
+      cp "icons/hyperhive-''${size}.png" \
+         "$out/share/icons/hicolor/''${size}x''${size}/apps/hyperhive.png"
+    done
+    # Flat pixmaps fallback (48px)
+    mkdir -p "$out/share/pixmaps"
+    cp icons/hyperhive-48.png "$out/share/pixmaps/hyperhive.png"
+
+    # .desktop entries — NoDisplay so they don't show up in app launchers
+    # but are still discovered by process-to-icon matchers (GNOME, etc.).
+    mkdir -p "$out/share/applications"
+    cat > "$out/share/applications/hive-c0re.desktop" <<EOF
+[Desktop Entry]
+Type=Application
+Name=hyperhive core
+GenericName=hive-c0re
+Comment=hyperhive host daemon — container lifecycle, broker, operator dashboard
+Exec=hive-c0re serve
+Icon=hyperhive
+Categories=System;
+NoDisplay=true
+StartupNotify=false
+EOF
+    cat > "$out/share/applications/hive-ag3nt.desktop" <<EOF
+[Desktop Entry]
+Type=Application
+Name=hyperhive agent
+GenericName=hive-ag3nt
+Comment=hyperhive per-agent harness (claude turn loop + MCP server)
+Exec=hive serve
+Icon=hyperhive
+Categories=System;
+NoDisplay=true
+StartupNotify=false
+EOF
+    runHook postInstall
+  '';
+
+  dontFixup = true;
+
+  meta = {
+    description = "XDG icon set and .desktop entries for hyperhive processes";
+  };
+}