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>