feat: sync auto-next obligation gate hardening

docs/runbooks/approved-plan-continuity.md

@@ -35,6 +35,10 @@
 - Use this field to state whether the reply closed under a dispatch-linked continuation path or some separately defined terminal closure state.
 - This field is defined here as a receipt field only; legal closure states and gate enforcement are defined in later tasks.
 
+### `nextTaskId`
+- The identifier of the required next task when continuity depends on a same-plan auto-next transition.
+- Use this field only to prove that the receipt links to the exact next task that had to be dispatched.
+- This field is the minimal hardening field for next-task linkage; it prevents unrelated dispatches, checkpoints, or stale receipts from spoofing continuity pass.
 
 ## Legal terminal states