Add core enforcement overview

Author: Quinton Stackfield

core/README.md

@@ -0,0 +1,10 @@
+## Core Enforcement Logic
+
+This directory contains the **execution-time enforcement boundary**.
+
+- `executor.py` - the only location where state change is permitted
+- `authority_gate.py` - fail-closed authority validation
+- `artifact_exporter.py` - immutable execution evidence
+- `evaluation.py` - aggregate outcome analysis
+
+No code outside this directory can cause state change.