Scrubs model output of characters that break downstream systems.

Removes ASCII control characters other than tab, newline and carriage return (they break JSON decoding and Postgres's UTF-8 encoding), the DEL character, and any bytes that are not valid UTF-8. Applied to every text the retain extractor and the consolidation merge take from a model before it is stored. Port of Hindsight's sanitize_llm_output (engine/llm_wrapper.py).