Add service infrastructure for long-running deployment

- Add configuration system (config/*.exs, OrgGarden.Config)
- Refactor supervision tree with DynamicSupervisor and Registry
- Add OrgGarden.Server for serve mode lifecycle management
- Add health check HTTP endpoints (Bandit/Plug on :9090)
- Add telemetry events for export and watcher operations
- Implement graceful shutdown with SIGTERM handling
- Add Mix Release support with overlay scripts
- Add NixOS module for systemd service deployment
- Update documentation with service usage

flake.nix

@@ -7,6 +7,15 @@
   };
 
   outputs = { self, nixpkgs, flake-utils }:
+    let
+      # System-independent outputs
+      nixosModule = import ./nix/module.nix;
+    in
+    {
+      # NixOS module (system-independent)
+      nixosModules.default = nixosModule;
+      nixosModules.org-garden = nixosModule;
+    } //
     flake-utils.lib.eachDefaultSystem (system:
       let
         pkgs = import nixpkgs { inherit system; };
@@ -47,6 +56,8 @@
               ./mix.exs
               ./mix.lock
               ./lib
+              ./config
+              ./rel
             ];
           };
           escriptBinName = "org_garden";