feat: support disabling bundled system skills (#13792)

Support disable bundled system skills with a config:

[skills.bundled]
enabled = false

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

@@ -2681,6 +2681,10 @@ impl Config {
             .network
             .is_some()
     }
+
+    pub fn bundled_skills_enabled(&self) -> bool {
+        crate::skills::manager::bundled_skills_enabled_from_stack(&self.config_layer_stack)
+    }
 }
 
 pub(crate) fn uses_deprecated_instructions_file(config_layer_stack: &ConfigLayerStack) -> bool {