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/containers.nix

@@ -0,0 +1,23 @@
+{ config, lib, pkgs, ... }:
+
+let
+  cfg = config.nomarchy.system.containers;
+in
+{
+  config = lib.mkIf cfg.enable {
+    virtualisation.podman = {
+      enable = true;
+      # `docker` and `docker-compose` invocations transparently route to
+      # podman. Pairs cleanly with rootless mode.
+      dockerCompat = true;
+      defaultNetwork.settings.dns_enabled = true;
+    };
+
+    environment.systemPackages = with pkgs; [
+      podman
+      podman-compose
+      podman-tui
+      dive
+    ];
+  };
+}