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/telemetry.ex

@@ -0,0 +1,109 @@
+defmodule OrgGarden.Telemetry do
+  @moduledoc """
+  Telemetry event definitions and logging handler.
+
+  ## Events
+
+  The following telemetry events are emitted:
+
+    * `[:org_garden, :export, :start]` — Export of a single file started
+      - Metadata: `%{file: path}`
+
+    * `[:org_garden, :export, :stop]` — Export of a single file completed
+      - Measurements: `%{duration: native_time}`
+      - Metadata: `%{file: path}`
+
+    * `[:org_garden, :export, :exception]` — Export failed
+      - Measurements: `%{duration: native_time}`
+      - Metadata: `%{file: path, kind: kind, reason: reason}`
+
+    * `[:org_garden, :watcher, :file_processed]` — File change processed
+      - Metadata: `%{path: path, event: :created | :modified | :deleted}`
+
+    * `[:org_garden, :server, :start]` — Server started
+      - Metadata: `%{port: port}`
+
+    * `[:org_garden, :server, :stop]` — Server stopped
+      - Metadata: `%{reason: reason}`
+
+  ## Usage
+
+  Attach a handler to log events:
+
+      OrgGarden.Telemetry.attach_logger()
+
+  Or use `:telemetry.attach/4` for custom handling.
+  """
+
+  require Logger
+
+  @doc """
+  Attach a simple logging handler for telemetry events.
+  """
+  def attach_logger do
+    events = [
+      [:org_garden, :export, :stop],
+      [:org_garden, :export, :exception],
+      [:org_garden, :watcher, :file_processed],
+      [:org_garden, :server, :start],
+      [:org_garden, :server, :stop]
+    ]
+
+    :telemetry.attach_many(
+      "org-garden-logger",
+      events,
+      &handle_event/4,
+      nil
+    )
+  end
+
+  @doc """
+  Detach the logging handler.
+  """
+  def detach_logger do
+    :telemetry.detach("org-garden-logger")
+  end
+
+  # -------------------------------------------------------------------
+  # Event handlers
+  # -------------------------------------------------------------------
+
+  defp handle_event([:org_garden, :export, :stop], measurements, metadata, _config) do
+    duration_ms = System.convert_time_unit(measurements.duration, :native, :millisecond)
+    Logger.debug("Export completed: #{metadata.file} (#{duration_ms}ms)")
+  end
+
+  defp handle_event([:org_garden, :export, :exception], _measurements, metadata, _config) do
+    Logger.error("Export failed: #{metadata.file} - #{inspect(metadata.reason)}")
+  end
+
+  defp handle_event([:org_garden, :watcher, :file_processed], _measurements, metadata, _config) do
+    Logger.debug("Watcher processed: #{metadata.event} #{metadata.path}")
+  end
+
+  defp handle_event([:org_garden, :server, :start], _measurements, metadata, _config) do
+    Logger.info("Server started on port #{metadata.port}")
+  end
+
+  defp handle_event([:org_garden, :server, :stop], _measurements, metadata, _config) do
+    Logger.info("Server stopped: #{inspect(metadata.reason)}")
+  end
+
+  # -------------------------------------------------------------------
+  # Convenience functions for emitting events
+  # -------------------------------------------------------------------
+
+  @doc """
+  Wrap a function with export telemetry events.
+  """
+  def span_export(file, fun) when is_function(fun, 0) do
+    :telemetry.span(
+      [:org_garden, :export],
+      %{file: file},
+      fn ->
+        result = fun.()
+        {result, %{file: file}}
+      end
+    )
+  end
+end