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/oneshot.rs

@@ -14,7 +14,7 @@ use std::fs;
 use std::path::PathBuf;
 use std::sync::OnceLock;
 
-use super::api::{self, ApiClient, Message, Usage};
+use super::api::{ApiClient, Message, Usage};
 use super::tools::{self as agent_tools};
 use super::Agent;