Add plugin ID to skill analytics (#20923)

## Summary
- thread plugin skill roots through the skills loader with their plugin
ID
- store plugin ID on loaded skill metadata for plugin-provided skills
- include plugin ID on skill invocation analytics events

## Test plan
- cargo check -p codex-core-skills
- cargo check -p codex-core -p codex-core-plugins -p codex-analytics
- cargo check -p codex-tui
- cargo check -p codex-plugin -p codex-core -p codex-core-plugins -p
codex-analytics
- cargo check -p codex-app-server
- cargo test -p codex-analytics
- HOME=/private/tmp/codex-empty-home cargo test -p codex-core-skills
- just fix -p codex-core-skills
- just fix -p codex-analytics
- just fix -p codex-core-plugins
- just fix -p codex-core
- just fmt
- git diff --check

codex-rs/core/src/agent/role_tests.rs

@@ -657,7 +657,7 @@ enabled = false
         SkillsManager::new(home.path().abs(), /*bundled_skills_enabled*/ true);
     let plugins_input = config.plugins_config_input();
     let plugin_outcome = plugins_manager.plugins_for_config(&plugins_input).await;
-    let effective_skill_roots = plugin_outcome.effective_skill_roots();
+    let effective_skill_roots = plugin_outcome.effective_plugin_skill_roots();
     let skills_input = skills_load_input_from_config(&config, effective_skill_roots);
     let outcome = skills_manager
         .skills_for_config(