Expose wayland-sessions at the FHS path moongreet expects

moongreet hardcodes /usr/share/wayland-sessions in src/sessions.rs and
greeter.rs calls get_sessions(None, None), so the default cannot be
overridden from config.

modules/greetd.nix

@@ -39,6 +39,17 @@ in
   # boots into multi-user.target, where the unit never starts.
   systemd.defaultUnit = "graphical.target";
 
+  # moongreet scans /usr/share/wayland-sessions and /usr/share/xsessions;
+  # both paths are hardcoded in src/sessions.rs and are not overridden by
+  # the caller in greeter.rs. NixOS collects session files under
+  # /run/current-system/sw/share instead, so the FHS paths are linked there.
+  environment.pathsToLink = [ "/share/wayland-sessions" "/share/xsessions" ];
+
+  systemd.tmpfiles.rules = [
+    "d /usr/share 0755 root root -"
+    "L+ /usr/share/wayland-sessions - - - - /run/current-system/sw/share/wayland-sessions"
+  ];
+
   # The greeter session spawns `moongreet` by name from niri-greeter.kdl,
   # so it has to resolve in PATH.
   environment.systemPackages = [ moonarchPkgs.moongreet ];