fix: book_chapter spawn context - use {character_profiles|} fallback to handle missing var

Tasks fail at step 7:package when character_profiles is not in context_vars because the
spawn context check incorrectly reports {character_profiles} as an unresolved placeholder.

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

templates/book_chapter.yml

@@ -1,6 +1,7 @@
 name: book_chapter
 description: "Write one chapter — continuity check, draft, self-check, then spawn editorial review."
 debug: true
+rag_exclude: [business_plan, charter]
 system: agent_prompt
 
 requires:
@@ -444,7 +445,7 @@ steps:
           chapter_ref: "{chapter_ref}"
           chapter_number: "{chapter_number}"
           review_letter: "a"
-          character_profiles: "{character_profiles}"
+          character_profiles: "{character_profiles|}"
 
       - task_type: chapter_review
         task_name: "Review (Lane): {chapter_ref}"
@@ -459,7 +460,7 @@ steps:
           chapter_ref: "{chapter_ref}"
           chapter_number: "{chapter_number}"
           review_letter: "b"
-          character_profiles: "{character_profiles}"
+          character_profiles: "{character_profiles|}"
 
       - task_type: chapter_review
         task_name: "Review (Cora): {chapter_ref}"
@@ -474,7 +475,7 @@ steps:
           chapter_ref: "{chapter_ref}"
           chapter_number: "{chapter_number}"
           review_letter: "c"
-          character_profiles: "{character_profiles}"
+          character_profiles: "{character_profiles|}"
 
       - task_type: chapter_roundtable
         task_name: "Roundtable: {chapter_ref}"
@@ -490,7 +491,7 @@ steps:
           genre_audience: "{genre_audience}"
           chapter_ref: "{chapter_ref}"
           chapter_target_words: "{chapter_target_words}"
-          character_profiles: "{character_profiles}"
+          character_profiles: "{character_profiles|}"
         depends_on:
           - "Review (Devon): {chapter_ref}"
           - "Review (Lane): {chapter_ref}"