feat: integrate rollback sentinel as NixOS module

Add rollback-sentinel NixOS module that:
- Deploys sentinel-check.sh (inline) and nixos-rollback.sh (from file) as
  system packages
- Runs a boot-time systemd oneshot service after multi-user.target with
  configurable delay — checks Tier-1 services, triggers rollback on failure
- Runs a post-rebuild service via activation script after every
  nixos-rebuild switch
- Exposes options for tier1Services, tier2Services, tier3InfoServices,
  bootDelay, rollbackMode (set-default/rollback-now/dry-run), and
  enablePostRebuild

Module wired into flake.nix for lazyworkhorse and enabled in
configuration.nix with standard Tier-1/2 service lists and 120s delay.

flake.nix

@@ -61,6 +61,7 @@
               ./modules/nixos/services/open_code_server.nix
               ./modules/nixos/services/ollama_init_custom_models.nix
               ./modules/nixos/services/openclaw_node.nix
+              ./modules/nixos/services/rollback-sentinel.nix
               ./modules/nixos/security/ai-worker-restricted.nix
               ./users/gortium.nix
               ./users/ai-worker.nix