Render only captured prompt templates

docs/internal/runner.md

@@ -42,6 +42,8 @@ Ordinary preparation selects the full prompt definition and construction-time
 hash. It retains its early request-ID check before direct-session
 normalization. Referenced message content and static template syntax were
 captured when the engine was built.
+Rendering uses those captured parses, cloning them per call to bind request
+input helpers; it does not parse templates during execution.
 
 `InspectPrompt` selects captured metadata directly from the repository. It
 returns owned metadata without