Agenda user @lily has written up a great article on how to integrate Agenda with Ulysses and DEVONthink Pro using x-callback-urls:
So, now you can plan – and/or create documentation after the facts, for example for auditing purposes – your work with note creation dates and flexible reminders or calendar events of future and past project tasks.
By using the x-callback system, you can create an ecosystem for creating, managing and maintaining content of alls kinds, types and complexity with maximum flexibility.