feat: unified watch server under org-garden

org-garden/lib/org_garden/transform.ex

@@ -0,0 +1,48 @@
+defmodule OrgGarden.Transform do
+  @moduledoc """
+  Behaviour that all markdown transform modules must implement.
+
+  ## Callbacks
+
+  - `init/1`      — called once before processing; returns transform-specific state.
+                    Default implementation returns the opts map unchanged.
+  - `apply/3`     — called per .md file; returns the (possibly modified) content.
+  - `teardown/1`  — optional cleanup after all files are processed.
+
+  ## Example
+
+      defmodule MyTransform do
+        @behaviour OrgGarden.Transform
+
+        @impl true
+        def init(opts), do: %{some_state: opts[:value]}
+
+        @impl true
+        def apply(content, state, _opts) do
+          String.replace(content, "foo", state.some_state)
+        end
+      end
+  """
+
+  @doc "One-time initialisation. Returns opaque state passed to apply/3."
+  @callback init(opts :: map()) :: term()
+
+  @doc "Transform file content. Returns the (possibly modified) content string."
+  @callback apply(content :: String.t(), state :: term(), opts :: map()) :: String.t()
+
+  @doc "Optional cleanup after all files are processed."
+  @callback teardown(state :: term()) :: :ok
+
+  @optional_callbacks teardown: 1
+
+  defmacro __using__(_) do
+    quote do
+      @behaviour OrgGarden.Transform
+
+      @impl OrgGarden.Transform
+      def init(opts), do: opts
+
+      defoverridable init: 1
+    end
+  end
+end