Allowlist the SPAWNED_FROM template provenance marker

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: 2 people

scripts/validate_structure.py

@@ -32,6 +32,7 @@
     "LICENSE",
     "Makefile",
     "README.md",
+    "SPAWNED_FROM",  # provenance marker stamped into spawned templates
     "index.md",
     "logo.png",
     "reading-queue.md",