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)

justfile

@@ -0,0 +1,30 @@
+default:
+    @just --list
+
+# ── Antigravity ────────────────────────────────────────────
+
+# Run antigravity (FHS-wrapped) from the attic cache
+antigravity-run:
+    nix run \
+        --extra-substituters "https://nix-cache-custom.oleks.space/attic-infra-cache-k3s-1" \
+        --extra-trusted-public-keys "attic-infra-cache-k3s-1:qYSNK3DmttQXCFqn1t50qoWGtQNPRFWq9mgQjD05DeU=" \
+        git+https://git.oleks.space/oleks/flake-hub#google-antigravity
+
+# Run antigravity (non-FHS variant) from the attic cache
+antigravity-run-no-fhs:
+    nix run \
+        --extra-substituters "https://nix-cache-custom.oleks.space/attic-infra-cache-k3s-1" \
+        --extra-trusted-public-keys "attic-infra-cache-k3s-1:qYSNK3DmttQXCFqn1t50qoWGtQNPRFWq9mgQjD05DeU=" \
+        git+https://git.oleks.space/oleks/flake-hub#google-antigravity-no-fhs
+
+# Build antigravity (FHS-wrapped)
+antigravity-build:
+    nix build .#google-antigravity --print-build-logs
+
+# Build antigravity (non-FHS variant)
+antigravity-build-no-fhs:
+    nix build .#google-antigravity-no-fhs --print-build-logs
+
+# Pull the latest jacopone/antigravity-nix pin
+antigravity-update:
+    nix flake update antigravity-nix