Clarify config layer iteration APIs (#36774)

## What changed

- Replace the ordering enum and `include_disabled` boolean with named
  `ConfigLayerStack` iterators for each precedence direction.
- Keep enabled-only iteration separate from `all_layers_*` iteration, which
  includes disabled layers.
- Update config consumers to use the iterator matching their precedence and
  disabled-layer requirements without allocating an intermediate `Vec`.

## Testing

- Add coverage that verifies ordering and disabled-layer filtering for all four
  iterators.

GitOrigin-RevId: d14df3db4a2eae80ba97cdec246bff405e6c5f3d

codex-rs/core/src/agents_md.rs

@@ -19,7 +19,6 @@ use crate::config::Config;
 use crate::context::UserInstructions as ContextUserInstructions;
 use crate::environment_selection::TurnEnvironmentSnapshot;
 use codex_config::ConfigLayerSource;
-use codex_config::ConfigLayerStackOrdering;
 use codex_config::default_project_root_markers;
 use codex_config::merge_toml_values;
 use codex_config::project_root_markers_from_config;
@@ -160,10 +159,7 @@ async fn agents_md_paths(
     let dir = cwd.clone();
 
     let mut merged = TomlValue::Table(toml::map::Map::new());
-    for layer in config.config_layer_stack.get_layers(
-        ConfigLayerStackOrdering::LowestPrecedenceFirst,
-        /*include_disabled*/ false,
-    ) {
+    for layer in config.config_layer_stack.layers_low_to_high() {
         if matches!(layer.name, ConfigLayerSource::Project { .. }) {
             continue;
         }