agent: bundle sampling fields as SamplingParams on AgentState

Collapse the split `temperature` / `top_p` / `top_k` fields on
AgentState into a single `sampling: SamplingParams` struct, mirroring
how the wire-level fields flow into the Generate RPC. Adds
`max_tokens` to SamplingParams so it's actually plumbed end to end
(previously the client had a hardcoded 4096 fallback inside
`run_session_generate`).

AgentState construction sites now set `sampling: SamplingParams { ...
max_tokens: 4096 }` as the default. The assignment sites in
oneshot.rs / subconscious.rs / unconscious.rs switch from
`st.temperature = X` to `st.sampling.temperature = X`.

`stream_session_mm` takes `SamplingParams` directly; the
`sampling_max_tokens()` helper goes away. `pb::GenerateRequest` is
populated with `sampling.max_tokens` (and the other fields) in
`run_session_generate`. SamplingParams is `pub` so it can be
embedded in the public AgentState without a visibility warning.

Co-Authored-By: Proof of Concept <poc@bcachefs.org>

src/agent/api/mod.rs

@@ -38,6 +38,21 @@ pub struct ReadoutManifest {
 /// from pairing with the manifest fetched at startup.
 pub type TokenReadout = Vec<Vec<f32>>;
 
+/// Client-side sampling state. Mirrors the wire-level fields in
+/// `GenerateRequest` (proto flattened its `SamplingParams` submessage
+/// in so the server handler reads them directly), but stays as a
+/// grouped struct on the client because UI / config / tests pass
+/// these around together.
+#[derive(Clone, Copy)]
+pub struct SamplingParams {
+    pub temperature: f32,
+    pub top_p: f32,
+    pub top_k: u32,
+    /// Decode budget. 0 = prefill only; >0 = decode up to this many
+    /// tokens, stopping early on EOS / stop_token_ids.
+    pub max_tokens: u32,
+}
+
 /// A JoinHandle that aborts its task when dropped.
 pub(crate) struct AbortOnDrop(tokio::task::JoinHandle<()>);
 
@@ -47,13 +62,6 @@ impl Drop for AbortOnDrop {
     }
 }
 
-/// Sampling parameters for model generation.
-#[derive(Clone, Copy)]
-pub(crate) struct SamplingParams {
-    pub temperature: f32,
-    pub top_p: f32,
-    pub top_k: u32,
-}
 
 // ─────────────────────────────────────────────────────────────
 //  Stream events — yielded by backends, consumed by the runner
@@ -288,13 +296,12 @@ async fn run_session_generate(
     } else {
         Vec::new()
     };
-    let max_tokens = sampling_max_tokens(&sampling);
     let req = pb::GenerateRequest {
         session_id: handle.session_id.clone(),
         append_tokens: pending,
         offset: handle.committed_len,
         truncating: false,
-        max_tokens,
+        max_tokens: sampling.max_tokens,
         logprobs_ranges: Vec::new(),
         logprob_top_k: 0,
         readout_ranges,
@@ -422,10 +429,4 @@ async fn flush_pending(
     Ok(())
 }
 
-fn sampling_max_tokens(_sampling: &SamplingParams) -> u32 {
-    // SamplingParams doesn't carry max_tokens today; 4096 mirrors
-    // the old server-side default and is a sensible interactive cap.
-    // TODO: plumb from the caller if we need bigger budgets.
-    4096
-}