Add prompt reference template functions

docs/internal/pipeline.md

@@ -44,6 +44,12 @@ CLI bindings resolve relative to the current working directory, and materialized
 reference content is passed to extractors through `ExtractionRequest`.
 Reference content is omitted from diagnostics and manifests.
 
+Prompt bundles can declare reference slots and use `reference` and
+`hasreference` template functions. Bundle loading validates string-literal slot
+names against the declaration. Rendering receives a lane reference set from the
+caller; unbound optional slots render as empty strings, and `hasreference`
+returns true only when at least one bound item has content.
+
 ## Registries And Module Specs
 
 `pipeline.Registries` holds concrete constructors for execution. A