Document prompt sessions and concurrency defaults

docs/internal/pipeline.md

@@ -10,11 +10,11 @@ own durable output shapes. Concrete production extensions are covered by
 ## Boundary
 
 The pipeline framework accepts a resolved composition, registries, shared
-dependencies, input bytes, and state/debug collaborators. It returns logical
-output files, normalized artifacts, recorded rejections and warnings, manifest
-provenance, and checkpoint decisions. The CLI owns process arguments,
-configuration discovery, physical roots, and placement of returned output
-files.
+dependencies, input bytes, a supplied prompt session, and state/debug
+collaborators. It returns logical output files, normalized artifacts, recorded
+rejections and warnings, manifest provenance, and checkpoint decisions. The
+CLI owns process arguments, configuration discovery, session resolution,
+physical roots, and placement of returned output files.
 
 The framework has one fixed shape:
 
@@ -84,6 +84,10 @@ incompatible producer prevents the consumer step from starting.
 
 The runner validates its input, installs no-op state collaborators when none
 were supplied, and serially performs source parsing and chunk-plan selection.
+It transports the supplied session unchanged to prompt-facing operations and
+run-manifest metadata; it neither derives a session nor substitutes a parsed
+source document identifier. The public session contract is owned by the
+[CLI reference](../cli.md#run).
 An accepted plan is materialized into source-addressed chunks and passes the
 configured chunk validators before any lane runs. A chunk rejection is a
 recorded pipeline outcome: lanes do not start, but the output stage can encode