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/features/src/lib.rs

@@ -146,6 +146,8 @@ pub enum Feature {
     ShellSnapshot,
     /// Allow turns to start while selected executors are still starting.
     DeferredExecutor,
+    /// Use the current working directory for turn diff display paths.
+    CwdRelativeTurnDiffs,
     /// Enable runtime metrics snapshots via a manual reader.
     RuntimeMetrics,
     /// Enable startup memory extraction and file-backed memory consolidation.
@@ -874,6 +876,12 @@ pub const FEATURES: &[FeatureSpec] = &[
         stage: Stage::UnderDevelopment,
         default_enabled: false,
     },
+    FeatureSpec {
+        id: Feature::CwdRelativeTurnDiffs,
+        key: "cwd_relative_turn_diffs",
+        stage: Stage::UnderDevelopment,
+        default_enabled: false,
+    },
     FeatureSpec {
         id: Feature::JsRepl,
         key: "js_repl",

codex-rs/features/src/tests.rs

@@ -59,6 +59,26 @@ fn executor_capability_discovery_is_an_opt_in_map_feature() {
     assert!(features.enabled(Feature::ExecutorCapabilityDiscovery));
 }
 
+#[test]
+fn cwd_relative_turn_diffs_is_an_opt_in_map_feature() {
+    let mut features = Features::with_defaults();
+    assert!(!features.enabled(Feature::CwdRelativeTurnDiffs));
+
+    features.apply_map(&BTreeMap::from([(
+        "cwd_relative_turn_diffs".to_string(),
+        true,
+    )]));
+
+    assert!(features.enabled(Feature::CwdRelativeTurnDiffs));
+
+    features.apply_map(&BTreeMap::from([(
+        "cwd_relative_turn_diffs".to_string(),
+        false,
+    )]));
+
+    assert!(!features.enabled(Feature::CwdRelativeTurnDiffs));
+}
+
 #[test]
 fn default_enabled_features_are_stable() {
     for spec in crate::FEATURES {