Add cwd-relative turn diff paths (#39625)

## What changed

- Add the opt-in `cwd_relative_turn_diffs` feature.
- When enabled, render turn diff paths relative to each selected environment's
  working directory instead of the detected Git root.
- Preserve repository-root-relative paths when the feature is disabled.

## Testing

- Cover enabled and disabled behavior for nested working directories across
  supported originators.

GitOrigin-RevId: 8b4908706a82b01f33ccd035b0945f9a29fce26f

codex-rs/core/src/session/turn.rs

@@ -223,7 +223,27 @@ pub(crate) async fn run_turn(
     // Keep the exact model-visible state used by this turn and its inline compactions.
     let (world_state, display_roots) = tokio::join!(
         sess.record_context_updates_and_set_reference_context_item(first_step_context.as_ref()),
-        turn_diff_display_roots(first_step_context.as_ref()),
+        async {
+            if first_step_context
+                .turn
+                .config
+                .features
+                .enabled(Feature::CwdRelativeTurnDiffs)
+            {
+                first_step_context
+                    .environments
+                    .turn_environments()
+                    .map(|environment| {
+                        (
+                            environment.selection().environment_id,
+                            environment.cwd().clone(),
+                        )
+                    })
+                    .collect()
+            } else {
+                turn_diff_display_roots(first_step_context.as_ref()).await
+            }
+        },
     );
     let mut world_state = world_state?;