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-not in the superseded `docs/assets/panel-ui/`.
+not in the superseded `docs/assets/panel-ui/` (removed 2026-10-16).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)
 

docs/plans/panel-redesign.md

@@ -24,7 +24,8 @@ When an implementation and a mockup disagree, the implementation is wrong.
 
 The 2026-09-08 givens ("no component library, Web Awesome layout CSS only") are
 withdrawn. `docs/assets/panel-ui/` and the stage-1 directions `a/`, `b/`, `c/`
-are history, not a brief.
+were history, not a brief, and were removed on 2026-10-16 (`6099716`; they
+are in the git history).
 
 ## Goal