Fix: add agent_name to roundtable and polish spawns

- book_chapter.yml: spawn Roundtable task with agent_name: Devon
  (dispatcher requires at least one agent; Devon chairs the roundtable
  while rotate_participants still drafts all available editors)
- chapter_roundtable.yml: spawn Polish task with agent_name: Iris
  (Iris is the designated chapter polish author)

Previously: dispatch_create_task_skipped reason=no_agent for both tasks.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

templates/book_chapter.yml

@@ -134,6 +134,7 @@ steps:
 
       - task_type: chapter_roundtable
         task_name: "Roundtable: {chapter_ref}"
+        agent_name: Devon
         priority: 7
         context:
           chapter_text: "{chapter_text}"