feat: Tier 1 system features — snapper, hibernate, containers, libvirt, keyring

Five opt-in modules lifted from bernardo/nixos and adapted to Nomarchy's
nomarchy.system.* option namespace. All default off (except keyring which
defaults on); evaluation of the existing VM/ISO is unchanged when the
toggles are unset.

- core/system/snapper.nix: BTRFS timeline snapshots (5h/7d), nixos-rebuild-snap
  wrapper that pre-snaps before each switch using the running hostname.
  Auto-skips when / isn't BTRFS so impermanence/non-BTRFS hosts are safe.
- core/system/hibernate.nix: suspend-then-hibernate on lid/idle/power-key
  with configurable idleMinutes (default 30). Description warns swap is
  required.
- core/system/containers.nix: rootless Podman with dockerCompat + dns +
  podman-compose, podman-tui, dive. Better default than the docker daemon
  for a desktop distro.
- core/system/virtualization.nix: extends the existing uwsm/Hyprland file
  with a libvirt + virt-manager + OVMF branch behind
  nomarchy.system.virtualization.libvirt.enable.
- core/system/pam.nix: GNOME Keyring auto-unlock at SDDM/login/hyprlock
  plus gcr-ssh-agent so SSH keys flow through the keyring instead of a
  separate ssh-agent. Default on.
- core/system/options.nix: declares the five new options.
- core/system/default.nix: imports the four new files.
- installer/install.sh: surfaces all five toggles as commented one-liners
  in the "Optional Nomarchy modules" section of the generated system.nix.
  Verified via the existing dry-run / generator smoke test.

Verified each toggle lights up the right NixOS option (services.snapper,
logind IdleAction, virtualisation.podman/libvirtd, pam.sddm.enableGnomeKeyring)
via nix eval against extendModules. VM and live-ISO toplevels still build.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

core/system/virtualization.nix

@@ -1,6 +1,11 @@
-{ lib, ... }:
+{ config, lib, pkgs, ... }:
 
+let
+  libvirt = config.nomarchy.system.virtualization.libvirt.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 = {
@@ -8,4 +13,14 @@
       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.
+  virtualisation.libvirtd.enable = lib.mkIf libvirt true;
+  environment.systemPackages = lib.mkIf libvirt (with pkgs; [
+    virt-manager
+    qemu
+    OVMF
+  ]);
 }