Add version date policy for cross-project documentation

REPO_POLICIES.md

@@ -1,5 +1,11 @@
 # Development Policies
 
+Version: 2026-02-22
+
+- Cross-project documentation (such as this file) must include a
+  `Version: YYYY-MM-DD` line near the top so it can be kept in sync
+  with the authoritative source as policies evolve.
+
 - All references to Docker images, Go modules, and packages must use
   cryptographic hashes. Mutable references (tags, `@latest`, etc.) are
   remote code execution vulnerabilities.