fix(workflow): use a DMN service task, not businessRuleTask (S-13, refs #14)

flowable-rest's businessRuleTask parse handler binds the legacy Drools/KIE
implementation (NoClassDefFoundError: org.kie.api...AgendaFilter at deploy time),
so registratie.bpmn failed to deploy and flowable-init exited 22. Flowable's
supported inline-DMN construct is a serviceTask with flowable:type="dmn" — switch
to it. The DMN cross-deploy via the process repository (previous commit) was fine;
this was the BPMN parse. Docs/comments updated to match.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

workflows/diploma-eligibility.dmn

@@ -1,6 +1,6 @@
 <?xml version="1.0" encoding="UTF-8"?>
 <!-- Diploma-eligibility decision (S-13, ADR-0016). Evaluated inline by the registratie process as a
-     BPMN businessRuleTask: given the diploma's origin, it sets the `route` the process should take.
+     BPMN DMN service task: given the diploma's origin, it sets the `route` the process should take.
      A foreign (Buitenlands) diploma routes through the extra CBGV-advies assessment step; a domestic
      one (or anything else) goes DIRECT to beoordeling. FIRST hit policy: the foreign rule wins, and
      the empty-input catch-all is the default. -->