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/user/thalamus.rs

@@ -35,12 +35,12 @@ impl ScreenView for ThalamusScreen {
                 KeyCode::Up => { self.sampling_selected = self.sampling_selected.saturating_sub(1); }
                 KeyCode::Down => { self.sampling_selected = (self.sampling_selected + 1).min(2); }
                 KeyCode::Right => {
-                    let delta = match self.sampling_selected {
+                    let _delta = match self.sampling_selected {
                         0 => 0.05, 1 => 0.05, 2 => 5.0, _ => 0.0,
                     };
                 }
                 KeyCode::Left => {
-                    let delta = match self.sampling_selected {
+                    let _delta = match self.sampling_selected {
                         0 => -0.05, 1 => -0.05, 2 => -5.0, _ => 0.0,
                     };
                 }