Correct retirement docs: deactivation clears future open instances

Retirement preserves the definition row, schedule rule, assignees,
windows and all durable completion history, but like any deactivation
it regenerates, so future uncompleted instances are cleared and none
are recreated while inactive. Fixes the "deletes nothing / all
instances preserved" wording in the core docstrings, the admin DELETE
route description, ADMIN-SPEC and the retire test's docstring
(review finding NQ-262-1). No behavioural change.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

api/routes_admin.py

@@ -208,8 +208,14 @@ retire/deactivate if there is"):
   :func:`nestquest_core.quest_definitions.delete_quest_definition`.  A
   definition with NO completion event on any instance is hard-deleted
   (instances, assignees, windows, the row and its unreferenced rule, in
-  one core transaction); one WITH history is retired by deactivation
-  and everything is preserved.  The response reports which happened
+  one core transaction); one WITH history is retired by deactivation.
+  Retirement preserves the definition row, schedule rule, assignees,
+  windows and ALL durable completion history; like any deactivation it
+  regenerates, so future UNCOMPLETED instances are cleared (rolling,
+  regenerable rows) and none are recreated while the definition is
+  inactive — the retired task stops generating and shows no open work.
+  Durable instance history lives only in the append-only
+  ``completion_events``.  The response reports which happened
   (``deleted`` / ``retired``) and carries the retired definition, or
   ``null`` when deleted.  An unknown id is 404.