Extracts the desktop_local_folder file-operation contract helpers:
* contract_enforcement_active: gates the contract on filesystem mode.
* evaluate_file_contract_outcome: scores tool outputs as success/no-op.
* log_file_contract: structured logging of contract verdicts.
This is the unit responsible for catching agents that claim to have
written/edited a file without actually invoking the filesystem tool.
Add-only; stream_new_chat.py keeps its inline duplicates until cutover.
