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

mix.exs

@@ -8,7 +8,8 @@ defmodule OrgGarden.MixProject do
       elixir: "~> 1.17",
       start_permanent: Mix.env() == :prod,
       deps: deps(),
-      escript: escript()
+      escript: escript(),
+      releases: releases()
     ]
   end
 
@@ -23,12 +24,27 @@ defmodule OrgGarden.MixProject do
     [main_module: OrgGarden.CLI]
   end
 
+  defp releases do
+    [
+      org_garden: [
+        include_executables_for: [:unix],
+        applications: [runtime_tools: :permanent],
+        steps: [:assemble, :tar]
+      ]
+    ]
+  end
+
   defp deps do
     [
       {:finch, "~> 0.19"},
       {:req, "~> 0.5"},
       {:jason, "~> 1.4"},
-      {:file_system, "~> 1.0"}
+      {:file_system, "~> 1.0"},
+      # Health check HTTP server
+      {:bandit, "~> 1.0"},
+      {:plug, "~> 1.15"},
+      # Telemetry
+      {:telemetry, "~> 1.0"}
     ]
   end
 end