Coordinate retry attempts with backend permits

docs/roadmap/implementation.md

@@ -279,6 +279,8 @@ next call. All existing wire tests must pass. Commit this refactoring separately
 
 ## Stage 4 — Implement The Internal Retry And Capacity Coordinator
 
+**Completed:** 2026-09-23.
+
 Create `internal/generation`, depending on domain, llm, and capacity. It implements
 the existing `llm.Client` consumed by the runner and repairer. For built-in
 generation it prepares once through Stage 3, then loops over capacity-controlled