feat(install): LUKS by default → auto-login, snapper snapshots, hibernation swapfile

- LUKS2 is the installer default; in exchange the generated config sets
  the new nomarchy.system.greeter.autoLogin (greetd initial_session) —
  the disk passphrase already gates the machine.
- @snapshots subvolume + nomarchy.system.snapper.enable: hourly/daily
  timeline snapshots of / and the nixos-rebuild-snap helper, ported from
  the previous iteration (3bdfc35), guarded to no-op on non-BTRFS roots.
- @swap subvolume with a swapfile sized to RAM by default (disko
  mkswapfile handles NOCOW); the installer computes the resume offset
  and wires boot.resumeDevice + resume_offset for hibernation.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

pkgs/nomarchy-install/default.nix

@@ -13,6 +13,7 @@
 , cryptsetup
 , lvm2             # dmsetup
 , pciutils         # lspci
+, btrfs-progs      # inspect-internal map-swapfile (hibernation offset)
   # Baked metadata — what this installer installs and where it came from.
 , templateDir              # templates/downstream (home.nix, theme-state.json)
 , nomarchyLock             # the distro's flake.lock (for offline lock composition)
@@ -51,7 +52,7 @@ stdenvNoCC.mkDerivation {
     wrapProgram $out/bin/nomarchy-install \
       --prefix PATH : ${lib.makeBinPath [
         bash gum disko whois git python3
-        util-linux gptfdisk parted cryptsetup lvm2 pciutils
+        util-linux gptfdisk parted cryptsetup lvm2 pciutils btrfs-progs
       ]} \
       --set NOMARCHY_INSTALL_SHARE "$share" \
       --set NOMARCHY_FLAKE_URL ${lib.escapeShellArg flakeUrl} \