fix(todo-continuation-enforcer): expose prune interval for cleanup

Prune interval created inside hook was not exposed for disposal,
preventing cleanup on plugin unload.

- Add dispose() method that clears the prune interval
- Export dispose in hook return type

Tests: 2 pass, 6 expects

src/hooks/todo-continuation-enforcer/index.ts

@@ -56,5 +56,6 @@ export function createTodoContinuationEnforcer(
     markRecovering,
     markRecoveryComplete,
     cancelAllCountdowns,
+    dispose: () => sessionStateStore.shutdown(),
   }
 }