feat: add support for project_root_markers in config.toml (#8359)

- allow configuring `project_root_markers` in `config.toml`
(user/system/MDM) to control project discovery beyond `.git`
- honor the markers after merging pre-project layers; default to
`[".git"]` when unset and skip ancestor walk when set to an empty array
- document the option and add coverage for alternate markers in config
loader tests

codex-rs/core/src/config_loader/tests.rs

@@ -355,3 +355,66 @@ async fn project_layer_is_added_when_dot_codex_exists_without_config_toml() -> s
 
     Ok(())
 }
+
+#[tokio::test]
+async fn project_root_markers_supports_alternate_markers() -> std::io::Result<()> {
+    let tmp = tempdir()?;
+    let project_root = tmp.path().join("project");
+    let nested = project_root.join("child");
+    tokio::fs::create_dir_all(project_root.join(".codex")).await?;
+    tokio::fs::create_dir_all(nested.join(".codex")).await?;
+    tokio::fs::write(project_root.join(".hg"), "hg").await?;
+    tokio::fs::write(
+        project_root.join(".codex").join(CONFIG_TOML_FILE),
+        "foo = \"root\"\n",
+    )
+    .await?;
+    tokio::fs::write(
+        nested.join(".codex").join(CONFIG_TOML_FILE),
+        "foo = \"child\"\n",
+    )
+    .await?;
+
+    let codex_home = tmp.path().join("home");
+    tokio::fs::create_dir_all(&codex_home).await?;
+    tokio::fs::write(
+        codex_home.join(CONFIG_TOML_FILE),
+        r#"
+project_root_markers = [".hg"]
+"#,
+    )
+    .await?;
+
+    let cwd = AbsolutePathBuf::from_absolute_path(&nested)?;
+    let layers = load_config_layers_state(
+        &codex_home,
+        Some(cwd),
+        &[] as &[(String, TomlValue)],
+        LoaderOverrides::default(),
+    )
+    .await?;
+
+    let project_layers: Vec<_> = layers
+        .layers_high_to_low()
+        .into_iter()
+        .filter_map(|layer| match &layer.name {
+            super::ConfigLayerSource::Project { dot_codex_folder } => Some(dot_codex_folder),
+            _ => None,
+        })
+        .collect();
+    assert_eq!(project_layers.len(), 2);
+    assert_eq!(project_layers[0].as_path(), nested.join(".codex").as_path());
+    assert_eq!(
+        project_layers[1].as_path(),
+        project_root.join(".codex").as_path()
+    );
+
+    let merged = layers.effective_config();
+    let foo = merged
+        .get("foo")
+        .and_then(TomlValue::as_str)
+        .expect("foo entry");
+    assert_eq!(foo, "child");
+
+    Ok(())
+}