Forward sessions through PromptKit requests

docs/integrations/pkg-promptkit.md

@@ -15,7 +15,7 @@ Notarius relies on the root `promptkit` package to:
 - construct an `Engine` with filesystem-backed prompt, schema, and optional
   profile sources;
 - prepare and run a `RunRequest` with named inline artifacts, variables,
-  prompt identity, and profile selection;
+  a direct session ID, prompt identity, and profile selection;
 - return rendered debug material, validated structured output, selected
   profile and model metadata, and token usage;
 - distinguish structured-output validation failure from execution failure; and
@@ -27,6 +27,12 @@ adapter boundary. It also retains responsibility for pipeline retries,
 scheduling, debug persistence, redaction, profile provenance, and conversion
 from private model responses into durable domain artifacts.
 
+Notarius sends its trimmed run session through PromptKit's direct session
+field, which is authoritative for provider session behavior. It also retains
+the same value as the `session_id` prompt variable for maintained prompt
+compatibility. Session IDs are stable, non-secret correlation identifiers and
+may be exposed to providers and provider observability.
+
 ## Notarius Ownership
 
 [LLM Runtime Internals](../internal/llm.md) describes how Notarius mounts