Demote the illustrative capability snapshot from a json fence to plain

Why: evidence tooling validates every ```json block in a task card as a receipt; the L1 summary card's illustrative capability snapshot (unquoted keys) is prose, not a receipt, and tripped a corrupt-receipt warning on every evidence run.
Evidence: after this change the card holds exactly one ```json block and it parses; content is byte-identical, only the fence marker changed.
Impact: Task card formatting only; no evidence content modified.
Refs: Dev PR #650 (card origin)

.task/final-cleanup-summary.md

@@ -28,7 +28,7 @@
 - `/api/admin/roles POST {"name":"developer"}` → `roleId: role_c106b6490383`
 - `/api/admin/users/bob/role PUT {"roleId":"role_xxx"}` → 200 OK
 - `/api/capability/explain` for admin → full `CapabilityExplain` DTO（U3A 输出）：
-  ```json
+  ```
   {
     "projectCreate": {allowed:true, source:"bootstrap-admin", linkedActions:[4 actions]},
     "memberBasicCollaborate": {4 actions, all "role-unassigned" for non-admin},