chore: untrack RENOVATE_PLAN.md

Working doc, kept locally via .gitignore. Removing from git also fixes the
markdown-lint failures it was causing.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Author: turegjorup

.gitignore

@@ -38,3 +38,6 @@ phpstan.neon
 
 .twig-cs-fixer.cache
 .playwright-mcp
+
+# Working docs not meant to ship
+RENOVATE_PLAN.md