chore: add ConfigLayerStack as a field of Config (#8448)

This should make it easier to load `.codex/skills/` and `.codex/rules/`
properly throughout.

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

@@ -52,7 +52,7 @@ impl ConfigLayerEntry {
     }
 }
 
-#[derive(Debug, Clone)]
+#[derive(Debug, Clone, Default, PartialEq)]
 pub struct ConfigLayerStack {
     /// Layers are listed from lowest precedence (base) to highest (top), so
     /// later entries in the Vec override earlier ones.