docs: drop the Cursor rule of the removed panel-ui mockups

.cursor/rules/panel-ui-system.mdc named docs/assets/panel-ui the source of
truth for mockups; that folder is gone (6099716) and the redesign's rule
(.cursor/rules/panel-design.mdc) is the one in force. The two plans that
mention the removed folders say they were removed.

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

docs/plans/inbound-antispam-panel.md

@@ -193,7 +193,7 @@ Filter lists** page (global-only: the lists are instance-wide) — see
 [panel-redesign.md](panel-redesign.md) § Room for roadmap items. The journal
 table has filters (decision, domain, date range). Mockups are drawn first, in
 `docs/assets/panel-redesign/panel/src/` under that plan's design contract —
-not in the superseded `docs/assets/panel-ui/`.
+not in the superseded `docs/assets/panel-ui/` (removed 2026-10-16).
 
 ## Schema (migration `0002_inbound_spam_log.sql`, the first after the 2.0 baseline)