feat: add xontribs, hyprspace, and gcc15-fixes overlay

- packages/xontribs.nix: xontrib-prompt-starship, -broot, -term-integrations
  wheels for use with `programs.xonsh.extraPackages` (or xonsh.override)
- packages/hyprspace.nix + hyprspace flake input (flake=false): rebuild
  plugin against the consumer's hyprland; exposed via overlays.hyprspace
- overlays/gcc15-fixes.nix: hotdoc/kitty/libsecret/xdg-desktop-portal/afdko
  workarounds so fleet nodes on the same pin can opt in with one line
- flake.nix: lift overlays out of eachSystem to the root (overlays.default
  was previously nested per-system, which doesn't match flake schema)

overlays/gcc15-fixes.nix

@@ -0,0 +1,43 @@
+# Narrow fixes for packages that fail under the fleet's pinned nixpkgs
+# with GCC 15 / current upstream breakage. Remove entries as upstream fixes
+# land so consumers can drop them on the next pin bump.
+final: prev: {
+  # hotdoc's bundled cmark declares `project(cmark VERSION 0.28.3)` without
+  # LANGUAGES, which makes CMake probe CXX; GCC 15 fails the probe. Adding
+  # LANGUAGES C skips the CXX check.
+  hotdoc = prev.hotdoc.overrideAttrs (old: {
+    postPatch = (old.postPatch or "") + ''
+      substituteInPlace cmark/CMakeLists.txt \
+        --replace-fail "project(cmark VERSION 0.28.3)" "project(cmark VERSION 0.28.3 LANGUAGES C)"
+    '';
+  });
+
+  # kitty's bundled base64 lib has incompatible pointer types that GCC 15
+  # promotes from warning to error.
+  kitty = prev.kitty.overrideAttrs (old: {
+    env = (old.env or { }) // {
+      NIX_CFLAGS_COMPILE = (old.env.NIX_CFLAGS_COMPILE or "") + " -Wno-error=incompatible-pointer-types";
+    };
+  });
+
+  # Skip D-Bus-dependent checks that fail under remote-builder sandboxes.
+  libsecret = prev.libsecret.overrideAttrs (_: {
+    doCheck = false;
+  });
+  xdg-desktop-portal = prev.xdg-desktop-portal.overrideAttrs (_: {
+    doCheck = false;
+  });
+
+  # afdko has two pre-existing test failures that block
+  # nototools → noto-fonts-color-emoji.
+  pythonPackagesExtensions = (prev.pythonPackagesExtensions or [ ]) ++ [
+    (pyFinal: pyPrev: {
+      afdko = pyPrev.afdko.overrideAttrs (old: {
+        disabledTests = (old.disabledTests or [ ]) ++ [
+          "test_non_varying_glyphs_bug356"
+          "test_ufo_contentsplist_parsing"
+        ];
+      });
+    })
+  ];
+}