feat: Use remote installed plugin cache for skills and MCP (#20096)

- Fetches and caches remote /installed plugin state
- Lets skills/list load skills from remote-installed cached plugins
without requiring a local marketplace entry
- Routes plugin list/startup/install/uninstall changes through async
plugin cache invalidation and MCP refresh

codex-rs/core/src/session/handlers.rs

@@ -23,7 +23,6 @@ use codex_config::CloudRequirementsLoader;
 use codex_config::LoaderOverrides;
 use codex_config::loader::load_config_layers_state;
 use codex_exec_server::LOCAL_FS;
-use codex_features::Feature;
 use codex_utils_absolute_path::AbsolutePathBuf;
 
 use crate::review_prompts::resolve_review_request;
@@ -619,10 +618,7 @@ pub async fn list_skills(sess: &Session, sub_id: String, cwds: Vec<PathBuf>, for
             }
         };
         let effective_skill_roots = plugins_manager
-            .effective_skill_roots_for_layer_stack(
-                &config_layer_stack,
-                config.features.enabled(Feature::Plugins),
-            )
+            .effective_skill_roots_for_layer_stack(&config_layer_stack, &config)
             .await;
         let skills_input = crate::SkillsLoadInput::new(
             cwd_abs.clone(),