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>

design/ADMIN-SPEC.md

@@ -91,9 +91,14 @@ otherwise it is retired (deactivated) and its history is kept." — with `Go bac
 danger-filled `Delete task`. Confirming calls `DELETE /api/v1/admin/quest-definitions/{id}`
 (admin plane only; the kids' panel has no equivalent control). Hybrid outcome (D-017): no
 completion event on any instance → the definition, its instances, assignees, windows and
-unreferenced rule are hard-deleted and the row leaves the list; otherwise nothing is
-deleted, the definition is deactivated and the row stays, shown inactive. An error keeps
-the sheet open with the API's detail.
+unreferenced rule are hard-deleted and the row leaves the list; otherwise the definition
+is retired (deactivated) and the row stays, shown inactive. 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 (they are
+rolling, regenerable rows) and none are recreated while the definition is inactive — the
+retired task stops generating and no longer shows open work. Durable instance history lives
+only in the append-only `completion_events`. An error keeps the sheet open with the API's
+detail.
 
 ### 3.3 Icon picker
 Each definition carries one optional icon, shown on both surfaces. The picker offers three