Add an override to skip project configuration (#38647)

## What changed

- Add `LoaderOverrides::ignore_project_config` to bypass project-root discovery and all project configuration layers.
- Keep other configuration sources, including session overrides and cloud configuration, active when the override is set.

## Testing

- Add a config loader test that verifies project configuration is not loaded or parsed while session and cloud values remain effective.

GitOrigin-RevId: b9aa44de526354fad6b900d6ae8d9b5fb8f700bb

codex-rs/config/src/loader/mod.rs

@@ -182,6 +182,7 @@ pub async fn load_config_layers_state(
     let active_user_profile = overrides.user_config_profile.clone();
     let ignore_managed_requirements = overrides.ignore_managed_requirements;
     let ignore_user_config = overrides.ignore_user_config;
+    let ignore_project_config = overrides.ignore_project_config;
     let ignore_user_and_project_exec_policy_rules =
         overrides.ignore_user_and_project_exec_policy_rules;
     let mut requirements_layers = Vec::new();
@@ -353,7 +354,7 @@ pub async fn load_config_layers_state(
     }
 
     let mut startup_warnings = None;
-    if let Some(cwd) = cwd {
+    if !ignore_project_config && let Some(cwd) = cwd {
         let mut merged_so_far = TomlValue::Table(toml::map::Map::new());
         for layer in &layers {
             merge_toml_values(&mut merged_so_far, &layer.config);

codex-rs/config/src/state.rs

@@ -52,6 +52,8 @@ pub struct LoaderOverrides {
     /// Remote app servers own their authentication policy independently.
     pub ignore_login_requirements: bool,
     pub ignore_user_config: bool,
+    /// Skip project-root discovery and all project configuration layers.
+    pub ignore_project_config: bool,
     pub ignore_user_and_project_exec_policy_rules: bool,
     //TODO(gt): Add a macos_ prefix to this field and remove the target_os check.
     #[cfg(target_os = "macos")]
@@ -75,6 +77,7 @@ impl LoaderOverrides {
             ignore_managed_requirements: false,
             ignore_login_requirements: false,
             ignore_user_config: false,
+            ignore_project_config: false,
             ignore_user_and_project_exec_policy_rules: false,
             #[cfg(target_os = "macos")]
             managed_preferences_base64: Some(String::new()),

codex-rs/core/src/config/config_loader_tests.rs

@@ -218,6 +218,68 @@ invalid = ["#,
     Ok(())
 }
 
+#[tokio::test]
+async fn ignore_project_config_skips_project_discovery() -> std::io::Result<()> {
+    let tmp = tempdir().expect("tempdir");
+    let codex_home = tmp.path().join("home");
+    let workspace = tmp.path().join("workspace");
+    tokio::fs::create_dir_all(codex_home.as_path()).await?;
+    tokio::fs::create_dir_all(workspace.join(".git")).await?;
+    make_config_for_test(
+        &codex_home,
+        &workspace,
+        TrustLevel::Trusted,
+        /*project_root_markers*/ None,
+    )
+    .await?;
+    let project_config_dir = workspace.join(".codex");
+    tokio::fs::create_dir_all(&project_config_dir).await?;
+    tokio::fs::write(
+        project_config_dir.join(CONFIG_TOML_FILE),
+        r#"model = "from-project"
+invalid = ["#,
+    )
+    .await?;
+
+    let cwd = AbsolutePathBuf::from_absolute_path(&workspace)?;
+    let layers = load_config_layers_state(
+        LOCAL_FS.as_ref(),
+        &codex_home,
+        Some(cwd),
+        &[(
+            "model".to_string(),
+            TomlValue::String("from-session".to_string()),
+        )],
+        ConfigLoadOptions {
+            loader_overrides: LoaderOverrides {
+                ignore_project_config: true,
+                ..LoaderOverrides::without_managed_config_for_tests()
+            },
+            cloud_config_bundle: CloudConfigBundleFixture::loader_with_enterprise_config(
+                r#"review_model = "from-cloud""#,
+            ),
+            ..Default::default()
+        },
+        &codex_config::NoopThreadConfigLoader,
+    )
+    .await?;
+
+    assert!(
+        layers
+            .layers_low_to_high()
+            .all(|layer| !matches!(layer.name, ConfigLayerSource::Project { .. }))
+    );
+    assert_eq!(
+        layers.effective_config().get("model"),
+        Some(&TomlValue::String("from-session".to_string()))
+    );
+    assert_eq!(
+        layers.effective_config().get("review_model"),
+        Some(&TomlValue::String("from-cloud".to_string()))
+    );
+    Ok(())
+}
+
 #[tokio::test]
 async fn ignore_rules_marks_config_stack_for_exec_policy_rule_skip() -> std::io::Result<()> {
     let tmp = tempdir().expect("tempdir");