Fix build warnings across workspace

- Remove redundant token fields from StreamEvent::Finished (data
  already delivered via Usage event)
- Remove dead hotkey_adjust_sampling, MAX_HISTORY, now()
- Fix unused variable warnings (delta, log)
- Suppress deserialization-only field warnings (jsonrpc, role)
- Make start_stream/chat_completion_stream_temp pub(crate)
- Remove unnecessary pub(crate) re-export of internal types

Remaining warnings are TODO items: SkipIndex (scoring not wired),
notify (MCP notifications not wired).

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

src/agent/api/openai.rs

@@ -181,14 +181,7 @@ pub(super) async fn stream_events(
     );
 
     let reason = finish_reason.unwrap_or_default();
-    let (pt, ct) = usage.as_ref()
-        .map(|u| (u.prompt_tokens, u.completion_tokens))
-        .unwrap_or((0, 0));
-    let _ = tx.send(StreamEvent::Finished {
-        reason,
-        prompt_tokens: pt,
-        completion_tokens: ct,
-    });
+    let _ = tx.send(StreamEvent::Finished { reason });
 
     Ok(())
 }