Expose HTTP transient retry control and preserve shared capacity

docs/internal/adapters.md

@@ -63,6 +63,9 @@ The handler enforces transport limits and strict JSON decoding before mapping
 DTOs into `promptkit.RunRequest`, `promptkit.ArtifactRef`, and
 `promptkit.ExecutionTargetOverride`. On success it reads Promptkit artifact,
 validation, model, usage, and metadata values directly.
+The optional HTTP retry count maps to `RunRequest.TransientRetries` without
+mutating the shared engine. Negative values are rejected before invocation;
+Promptkit controls retries and backend admission for concurrent requests.
 
 Failure mapping uses `errors.Is` against Promptkit's public sentinels and the
 HTTP reader's Scriptorium-owned containment and size errors. Wrapped reader