fix: roundtable uses all three editors as participants

Replace agent_name: Devon with agents: [Devon, Lane, Cora] so the
chapter_roundtable task has all three editors rotating in the debate.
Addresses user feedback that roundtable should not be a specific
agent - everyone in the editorial team should be drafted.

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

templates/book_chapter.yml

@@ -134,7 +134,10 @@ steps:
 
       - task_type: chapter_roundtable
         task_name: "Roundtable: {chapter_ref}"
-        agent_name: Devon
+        agents:
+          - Devon
+          - Lane
+          - Cora
         priority: 7
         context:
           chapter_text: "{chapter_text}"