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/pam.nix

@@ -0,0 +1,28 @@
+{ config, lib, ... }:
+
+let
+  cfg = config.nomarchy.system.keyring;
+in
+{
+  config = lib.mkIf cfg.enable {
+    # Auto-unlock GNOME Keyring at SDDM autologin and at login. hyprlock
+    # gets the same treatment so the session keyring stays unlocked when
+    # the screen lock disengages.
+    security.pam.services = {
+      login.enableGnomeKeyring = true;
+      sddm.enableGnomeKeyring = true;
+      hyprlock.enableGnomeKeyring = true;
+    };
+
+    # Run the keyring + the gcr SSH agent. Disabling `programs.ssh.startAgent`
+    # ensures keys flow through the keyring's agent (so unlock-on-login
+    # carries over to ssh) instead of a separate ssh-agent process.
+    services.gnome.gnome-keyring.enable = true;
+    services.gnome.gcr-ssh-agent.enable = true;
+    programs.ssh.startAgent = lib.mkForce false;
+
+    # Point downstream tooling at the gcr socket so `ssh` / `git` / etc.
+    # find the keyring's keys without per-user shell config.
+    environment.sessionVariables.SSH_AUTH_SOCK = "$XDG_RUNTIME_DIR/gcr/ssh";
+  };
+}