Build a combined prompt string as a fallback for backends that expect a single prompt.
If sys is empty, returns user as-is; otherwise formats as system+user sections.
Train a persistent global logit-bias adapter from prompt/assistant examples.
This does not modify base model weights; instead it learns a bias vector (size=vocab)
that is added to the logits during generation. The bias is saved to ADAPTER_DIR and
automatically loaded on startup and applied for all subsequent generations.