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

lib/org_garden/export.ex

@@ -16,6 +16,12 @@ defmodule OrgGarden.Export do
   """
   @spec export_file(String.t(), String.t(), String.t()) :: {:ok, non_neg_integer()} | {:error, term()}
   def export_file(orgfile, notes_dir, output_dir) do
+    OrgGarden.Telemetry.span_export(orgfile, fn ->
+      do_export_file(orgfile, notes_dir, output_dir)
+    end)
+  end
+
+  defp do_export_file(orgfile, notes_dir, output_dir) do
     section =
       orgfile
       |> Path.dirname()