template: make rename portable and add explicit otp_app dev flow

lib/my_plugin/plugin.ex

@@ -58,24 +58,44 @@ defmodule MyPlugin.Plugin do
   end
 
   defp manifest_path do
-    # In a release, manifest.json sits alongside ebin/ in the plugin directory.
-    # In dev mode (path dep), it's at the project root.
-    case :code.priv_dir(:my_plugin) do
-      {:error, :bad_name} ->
-        # Dev mode fallback: relative to project root
-        Path.join(__DIR__, "../../../manifest.json") |> Path.expand()
+    project_manifest = Path.join(__DIR__, "../../manifest.json") |> Path.expand()
 
-      priv_dir ->
-        priv_dir |> to_string() |> Path.join("../manifest.json") |> Path.expand()
-    end
+    candidates =
+      case :code.priv_dir(:my_plugin) do
+        {:error, :bad_name} ->
+          [project_manifest]
+
+        priv_dir ->
+          priv_dir = to_string(priv_dir)
+
+          [
+            Path.join(priv_dir, "../manifest.json") |> Path.expand(),
+            project_manifest
+          ]
+      end
+
+    first_existing_path(candidates) || project_manifest
   end
 
   defp migrations_path(manifest) do
     if manifest["migrations"] do
-      case :code.priv_dir(:my_plugin) do
-        {:error, :bad_name} -> nil
-        priv_dir -> priv_dir |> to_string() |> Path.join("repo/migrations")
-      end
+      candidates =
+        case :code.priv_dir(:my_plugin) do
+          {:error, :bad_name} ->
+            [Path.join(__DIR__, "../../priv/repo/migrations") |> Path.expand()]
+
+          priv_dir ->
+            [
+              Path.join(to_string(priv_dir), "repo/migrations") |> Path.expand(),
+              Path.join(__DIR__, "../../priv/repo/migrations") |> Path.expand()
+            ]
+        end
+
+      first_existing_path(candidates)
     end
   end
+
+  defp first_existing_path(paths) do
+    Enum.find(paths, &File.exists?/1)
+  end
 end