chore: Add rebrand assets, implementation guide, and gitignore local settings

Adds brand-assets/ (SVGs, brand guide) and REBRAND-IMPLEMENTATION-GUIDE.md
as reference docs. Removes .claude/settings.local.json from tracking
(contains local permissions and tokens).

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

.gitignore

@@ -206,6 +206,9 @@ marimo/_static/
 marimo/_lsp/
 __marimo__/
 
+# Claude Code (local settings with permissions/tokens)
+.claude/settings.local.json
+
 # Railway CLI (local tooling)
 node_modules/
 package.json