Derive collaboration settings from turn context (#32900)

## Why

`TurnContext` stored model and reasoning settings both as resolved turn fields and inside `CollaborationMode`, requiring the copies to stay synchronized when the model changed.

## What changed

- Store the mode and collaboration-mode developer instructions directly on `TurnContext`.
- Reconstruct `CollaborationMode` from those values plus the turn's current model and reasoning effort when callers need it.
- Read the mode directly for plan-mode checks, lifecycle events, and telemetry.

GitOrigin-RevId: b8fccafdd5d5cbccc84bc70d542588087ddf0093

codex-rs/core/src/compact_remote.rs

@@ -82,7 +82,7 @@ pub(crate) async fn run_remote_compact_task(
         trace_id: turn_context.trace_id.clone(),
         started_at: turn_context.turn_timing_state.started_at_unix_secs().await,
         model_context_window: turn_context.model_context_window(),
-        collaboration_mode_kind: turn_context.collaboration_mode.mode,
+        collaboration_mode_kind: turn_context.mode,
     });
     sess.send_event(&turn_context, start_event).await;