Follow-up to #87400: drop the max_iterations and prompt_file knobs from
auxiliary.background_review. The aux model routing (provider/model/
base_url/...) predates #87400 and stays; the enabled switch and the
usage telemetry stay. The fork's iteration budget returns to the
historical hardcoded 16.
Persist fork token usage under session_model_usage task=background_review,
emit a per-fork completion log line, and expose enabled/max_iterations/
prompt_file so operators can see and bound the automatic review cost.
Address review feedback: load auxiliary.background_review once per spawn,
classify completion logs by summarize action prefixes, treat explicit
api_call_count=None as the documented default of 1, and WARNING on the
fail-open enabled-gate path.
Teknium