fix: use single-brace {agent_slug} in chapter_review filename

Double-brace {{agent_slug}} left outer braces in filename even after
substitution, causing all 3 reviewers (Devon/Lane/Cora) to write to
the same file review-ch-XX-{agent-slug}.md overwriting each other.
Single-brace now correctly produces review-ch-XX-devon.md etc.

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

templates/chapter_review.yml

@@ -34,7 +34,7 @@ steps:
         3. VERDICT (pass / revise / rewrite — and why)
 
   - type: document
-    filename: "review-{chapter_ref}-{{agent_slug}}"
+    filename: "review-{chapter_ref}-{agent_slug}"
 
   - type: close
     rag_update: false