docs(releases): drop reserved word "role" from v15 kanban note

ADR-0090 D3 makes "role" a reserved-forbidden word in hand-written docs,
enforced by the check:role-word ratchet. #3079 introduced a generic-English
"the `stageField` role" in content/docs/releases/v15.mdx that is not in the
baseline, breaking the ratchet on main. Reword to "the `stageField` setting"
— the sentence is about which field drives lanes, not authz — so no baseline
entry is needed.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

Author: os-zhuang

content/docs/releases/v15.mdx

@@ -410,7 +410,7 @@ The pinned Console picks up the objectui 14.1 line (94 commits). Highlights:
   inline (#2568); the connector picker lists runtime-dispatchable connectors and
   marks declarative instances (#2563); positions as approver/escalateTo references
   (#2515).
-- **Kanban:** lanes honor the ADR-0085 `stageField` role, cards default to
+- **Kanban:** lanes honor the ADR-0085 `stageField` setting, cards default to
   `highlightFields`, conditional formatting accepts CEL rules (#2596/#2541/#2550).
 - **Record header:** metadata-driven multi-button layout (3 desktop / 1 mobile,
   `order` + `variant:'primary'` tie-break, ⋯ overflow menu) (#2574).