Document and demonstrate execution controls

docs/internal/runner.md

@@ -209,6 +209,16 @@ mapping and after the runner's deferred admission release. Preparation and
 prepared execution follows the same terminal mapping after a successful claim;
 invalid or discarded handles do not acquire an observer.
 
+The facade establishes an optional operation scope for ordinary runs before
+observation and preparation. A claimed prepared execution establishes its own
+scope from the retained duration; preparation and idle time do not consume it.
+The shared scope finalizer marks only a context failure caused by that scope's
+own deadline, preserving its underlying operation and cause. The scope is
+released before terminal delivery. At the shared prepared execution boundary,
+the runner conditionally invokes the adapter for the actual prepared snapshot
+while admission is held, with context checks before and after notification.
+The facade adapter projects a detached public value only when selected.
+
 Validation preparation and execution honor cancellation at every
 Promptkit-controlled boundary and do not publish a partial plan or result.
 Schema reads are bounded and context-checked between chunks; JSON decoding,