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/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 withdrawn.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