Clean up warnings: StreamToken pub, dead oneshot code, SkipIndex

Made StreamToken pub (was pub(crate), needed by context.rs).
Removed dead API_CLIENT, get_client, sampling/priority fields
from oneshot. Suppressed pre-existing SkipIndex warning in learn.rs.

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

src/agent/api/mod.rs

@@ -44,7 +44,7 @@ pub(crate) struct SamplingParams {
 // ─────────────────────────────────────────────────────────────
 
 /// One token from the streaming completions API.
-pub(crate) enum StreamToken {
+pub enum StreamToken {
     Token { text: String, id: u32 },
     Done { usage: Option<Usage> },
     Error(String),