agent: add sampling parameters (temperature, top_p, top_k)

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>

src/subconscious/api.rs

@@ -76,12 +76,17 @@ pub async fn call_api_with_tools(
         let mut msg_opt = None;
         let mut usage_opt = None;
         for attempt in 0..5 {
+            let sampling = crate::agent::api::SamplingParams {
+                temperature: temperature.unwrap_or(0.6),
+                top_p: 0.95,
+                top_k: 20,
+            };
             match client.chat_completion_stream_temp(
                 &messages,
                 Some(&tool_defs),
                 &ui_tx,
                 &reasoning,
-                temperature,
+                sampling,
                 Some(priority),
             ).await {
                 Ok((msg, usage)) => {