refactor(system): move programs.uwsm to its own session.nix

The session-manager wiring (uwsm + the Hyprland Wayland-compositor
entry that gives Hyprland a proper graphical-session.target so user
services like nomarchy-wallpaper, walker, and elephant chain off it)
had lived in core/system/virtualization.nix by historical accident —
loaded unconditionally on every install, nothing to do with libvirt
or docker.

Lifted into a dedicated core/system/session.nix and imported from
core/system/default.nix between systemd.nix and virtualization.nix.
virtualization.nix now contains only the libvirt + docker branches
its filename implies.

`nix flake check --no-build` clean. No behaviour change.

core/system/virtualization.nix

@@ -5,16 +5,6 @@ let
   docker = config.nomarchy.system.virtualization.docker.enable;
 in
 {
-  # uwsm + Hyprland session — present on every Nomarchy install regardless
-  # of the optional libvirt branch below.
-  programs.uwsm = {
-    enable = lib.mkDefault true;
-    waylandCompositors.hyprland = {
-      binPath = "/run/current-system/sw/bin/Hyprland";
-      prettyName = "Hyprland";
-    };
-  };
-
   # Optional: libvirt + virt-manager + OVMF. Toggle with
   # `nomarchy.system.virtualization.libvirt.enable = true;`. The user must
   # be in the `libvirtd` group to drive virsh / virt-manager.