Move temperature from a per-call parameter to an Agent field,
add top_p and top_k. All three are sent to the API via a new
SamplingParams struct, displayed on the F5 thalamus screen.
Defaults: temperature=0.6, top_p=0.95, top_k=20 (Qwen3.5 defaults).
Also adds top_p and top_k to ChatRequest so they're sent in the
API payload. Previously only temperature was sent.
UI controls for adjusting these at runtime are not yet implemented.
Co-Authored-By: Proof of Concept <poc@bcachefs.org>
