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)

ci/build.py

@@ -39,6 +39,8 @@ run(f"'{attic}' login ci {ATTIC_SERVER} '{ATTIC_TOKEN}'")
 
 # Packages per arch
 packages = ["hello-world", "geesefs", "xonsh"]
+if ARCH == "x86_64-linux":
+    packages += ["google-antigravity", "google-antigravity-no-fhs"]
 if ARCH == "s390x-linux":
     packages += ["attic-client"]