fix pre-commit EOF fixer regression and clarify test SOP

Author: rookiestar28

scripts/precommit_black_single.py

@@ -32,4 +32,3 @@ def main(argv: list[str]) -> int:
 
 if __name__ == "__main__":
     raise SystemExit(main(sys.argv))
-

tests/TEST_SOP.md

@@ -18,6 +18,7 @@ pre-commit run detect-secrets --all-files
 ```bash
 pre-commit run --all-files --show-diff-on-failure
 ```
+If any hook reports “files were modified”, stage + commit those changes and re-run until this step is clean.
 
 3) Backend unit tests (recommended; CI enforces)
 ```bash