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

@@ -69,6 +69,23 @@
         "url": "https://git.oleks.space/oleks/fleet-pins"
       }
     },
+    "hyprspace": {
+      "flake": false,
+      "locked": {
+        "lastModified": 1764820995,
+        "narHash": "sha256-IMa4mvkF0w7OAy+yEzPFYs2an332K30lf5qfUOAS9Cw=",
+        "owner": "KZDKM",
+        "repo": "Hyprspace",
+        "rev": "0467be86b18cfc324fab04afbd40fe9ef80f7fa9",
+        "type": "github"
+      },
+      "original": {
+        "owner": "KZDKM",
+        "repo": "Hyprspace",
+        "rev": "0467be86b18cfc324fab04afbd40fe9ef80f7fa9",
+        "type": "github"
+      }
+    },
     "nixpkgs": {
       "locked": {
         "lastModified": 1771848320,
@@ -89,6 +106,7 @@
       "inputs": {
         "flake-utils": "flake-utils",
         "fleet-pins": "fleet-pins",
+        "hyprspace": "hyprspace",
         "nixpkgs": [
           "fleet-pins",
           "nixpkgs-projects"