feat(issue-34): a job records its parent; submit --parent-job (MAM_JOB_ID by default) and the brief tells the agent to pass it on

- registry: parent_job field, register --parent-job fails closed on an
  unknown parent, children_of/descendants_of, list --parent
- wrapper: --parent-job; $MAM_JOB_ID default (ignored with a warning when
  this registry lacks it); brief line and one sentence at the end of the
  injected instructions
- create_session: the onboarding instructions carry the same sentence

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

.agents/skills/multi-agent-mux-create/scripts/create_session.sh

@@ -642,7 +642,8 @@ On progress (optional): $pub --event progress --detail '<short status>'.
 On success run:      $pub --event completed --detail '<one-line summary>'.
 On failure run:      $pub --event error     --detail '<one-line reason>'.
 
-Task: read the complete task in $task_brief and follow it."
+Task: read the complete task in $task_brief and follow it.
+If you delegate any part of it with multi-agent-mux-delegate-job submit, add --parent-job $DELEGATE_JOB_ID so the orchestrator waits for it."
 
   # Inject instructions into the herdr pane
   rc=0