feat(games): add gamescope osu! session

programs/games.nix

@@ -1,4 +1,15 @@
-{ pkgs, ... }:
+{ lib, pkgs, ... }:
+let
+  osuSessionFile =
+    (pkgs.writeTextDir "share/wayland-sessions/osu.desktop" ''
+      [Desktop Entry]
+      Name=osu!
+      Comment=Free-to-win rhythm game
+      Exec=${lib.getExe pkgs.gamescope} -- ${lib.getExe pkgs.osu-lazer-bin}
+      Type=Application
+    '').overrideAttrs
+      { passthru.providedSessions = [ "osu" ]; };
+in
 {
   hardware = {
     xone.enable = true;
@@ -26,4 +37,6 @@
 
     gamemode.enable = true;
   };
+
+  services.displayManager.sessionPackages = [ osuSessionFile ];
 }