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/hibernate.nix

@@ -0,0 +1,24 @@
+{ config, lib, ... }:
+
+let
+  cfg = config.nomarchy.system.hibernation;
+in
+{
+  config = lib.mkIf cfg.enable {
+    # Wait this long after suspend before hibernating, and use the same
+    # delay as the idle-action timeout so the two paths agree.
+    systemd.sleep.extraConfig = ''
+      HibernateDelaySec=${toString (cfg.idleMinutes * 60)}
+    '';
+
+    services.logind = {
+      settings.Login = {
+        HandleLidSwitch = lib.mkDefault "suspend-then-hibernate";
+        HandleLidSwitchExternalPower = lib.mkDefault "suspend";
+        HandlePowerKey = "hibernate";
+        IdleAction = "suspend-then-hibernate";
+        IdleActionSec = toString (cfg.idleMinutes * 60);
+      };
+    };
+  };
+}