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)

flake.lock

@@ -1,5 +1,26 @@
 {
   "nodes": {
+    "antigravity-nix": {
+      "inputs": {
+        "flake-utils": "flake-utils",
+        "nixpkgs": [
+          "nixpkgs"
+        ]
+      },
+      "locked": {
+        "lastModified": 1776411505,
+        "narHash": "sha256-7HILfdTOvWzAMmfS3lQV3MDmVrr2Epj8x4c9F3mzOSE=",
+        "owner": "jacopone",
+        "repo": "antigravity-nix",
+        "rev": "9f31d28882185d30c0e3082fbc52dab1cd4e879f",
+        "type": "github"
+      },
+      "original": {
+        "owner": "jacopone",
+        "repo": "antigravity-nix",
+        "type": "github"
+      }
+    },
     "flake-utils": {
       "inputs": {
         "systems": "systems"
@@ -18,6 +39,24 @@
         "type": "github"
       }
     },
+    "flake-utils_2": {
+      "inputs": {
+        "systems": "systems_2"
+      },
+      "locked": {
+        "lastModified": 1731533236,
+        "narHash": "sha256-l0KFg5HjrsfsO/JpG+r7fRrqm12kzFHyUHqHCVpMMbI=",
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "rev": "11707dc2f618dd54ca8739b309ec4fc024de578b",
+        "type": "github"
+      },
+      "original": {
+        "owner": "numtide",
+        "repo": "flake-utils",
+        "type": "github"
+      }
+    },
     "fleet-pins": {
       "inputs": {
         "nixpkgs": "nixpkgs",
@@ -104,7 +143,8 @@
     },
     "root": {
       "inputs": {
-        "flake-utils": "flake-utils",
+        "antigravity-nix": "antigravity-nix",
+        "flake-utils": "flake-utils_2",
         "fleet-pins": "fleet-pins",
         "hyprspace": "hyprspace",
         "nixpkgs": [
@@ -127,6 +167,21 @@
         "repo": "default",
         "type": "github"
       }
+    },
+    "systems_2": {
+      "locked": {
+        "lastModified": 1681028828,
+        "narHash": "sha256-Vy1rq5AaRuLzOxct8nz4T6wlgyUR7zLU309k9mBC768=",
+        "owner": "nix-systems",
+        "repo": "default",
+        "rev": "da67096a3b9bf56a91d16901293e51ba5b49a27e",
+        "type": "github"
+      },
+      "original": {
+        "owner": "nix-systems",
+        "repo": "default",
+        "type": "github"
+      }
     }
   },
   "root": "root",