fix: harden plugin feature gating (#15104)

Resubmit https://github.com/openai/codex/pull/15020 with correct
content.

1. Use requirement-resolved config.features as the plugin gate.
2. Guard plugin/list, plugin/read, and related flows behind that gate.
3. Skip bad marketplace.json files instead of failing the whole list.
4. Simplify plugin state and caching.

codex-rs/core/src/tools/runtimes/shell/unix_escalation.rs

@@ -489,7 +489,7 @@ impl CoreShellActionProvider {
             .session
             .services
             .skills_manager
-            .skills_for_cwd(&self.turn.cwd, force_reload)
+            .skills_for_cwd(&self.turn.cwd, self.turn.config.as_ref(), force_reload)
             .await;
 
         let program_path = program.as_path();