Example of how to get a callback that only runs when Unity opens (not on script recompile)

OnOpenUnityEditorExample.cs

[InitializeOnLoad]
  public static class OnOpenUnityEditorExample {
		const string lastCompileTimeKey = "InkIntegrationLastCompileTime";

		static InkEditorUtils () {
      float lastCompileTime = LoadAndSaveLastCompileTime();
			if(EditorApplication.timeSinceStartup < lastCompileTime)
				OnOpenUnityEditor();
		}
		static void OnOpenUnityEditor () {
			InkLibrary.Rebuild();
		}
  
    static float LoadAndSaveLastCompileTime () {
			float lastCompileTime = 0;
			if(EditorPrefs.HasKey(lastCompileTimeKey))
				lastCompileTime = EditorPrefs.GetFloat(lastCompileTimeKey);
			EditorPrefs.SetFloat(lastCompileTimeKey, (float)EditorApplication.timeSinceStartup);
			return lastCompileTime;
		}
}