Update .Rbuildignore to exclude upgrade files

Exclude from package build:
- .venv (Python virtual environment)
- CLAUDE.md (project instructions)
- PLOTLYJS_UPGRADE_LOG.md (upgrade tracking)
- PLOTLYJS_UPGRADE_TODO.md (upgrade todos)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: cpsievert

.Rbuildignore

@@ -21,3 +21,7 @@ README.Rmd
 abbvie.R
 ^\.httr-oauth$
 ^\.github$
+^\.venv$
+CLAUDE\.md
+PLOTLYJS_UPGRADE_LOG\.md
+PLOTLYJS_UPGRADE_TODO\.md