CLAUDE.md: add macOS git clean instruction for sed backup files

Author: dannywillems

CLAUDE.md

@@ -424,5 +424,10 @@ When modifying CI workflows, especially for performance improvements:
 - **MANDATORY**: Run `make fix-trailing-whitespace` before every commit
 - **MANDATORY**: Run `make check-trailing-whitespace` to verify no trailing
   whitespaces remain
+- **macOS only**: After running `make fix-trailing-whitespace` or
+  `make format-md`, run these commands to clean up backup files created by sed:
+  ```bash
+  git clean -f -- '*-e' && git clean -f -- '**/*-e'
+  ```
 - This applies to ALL file modifications, regardless of file type
 - Trailing whitespaces are strictly prohibited in the codebase