Add planning docs to .gitignore

Keep todo.md, REGRESSION_PREVENTION.md, and TEST_DATA.md as local-only
development files to avoid cluttering the repository with planning docs.

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

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

Author: irskep

.gitignore

@@ -43,3 +43,8 @@ htmlcov/
 # OS
 .DS_Store
 Thumbs.db
+
+# Planning and development docs (keep local only)
+todo.md
+REGRESSION_PREVENTION.md
+TEST_DATA.md