Adds the IUserTaskClient port and implements it on the Workflow Client (the only code that talks to Flowable, §8.2): query open Beoordelen tasks (werkbak) with their registrationId, claim a task for a behandelaar, and complete it carrying the besluit into the process. Registered in DI for later wiring (S-12c). Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>