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-plugins/src/loader.rs

@@ -569,6 +569,7 @@ async fn load_plugin(
     loaded_plugin.skill_roots = plugin_skill_roots(&plugin_root, manifest_paths);
     let resolved_skills = load_plugin_skills(
         &plugin_root,
+        &loaded_plugin_id,
         manifest_paths,
         restriction_product,
         skill_config_rules,
@@ -647,6 +648,7 @@ impl ResolvedPluginSkills {
 
 pub async fn load_plugin_skills(
     plugin_root: &AbsolutePathBuf,
+    plugin_id: &PluginId,
     manifest_paths: &PluginManifestPaths,
     restriction_product: Option<Product>,
     skill_config_rules: &SkillConfigRules,
@@ -657,6 +659,7 @@ pub async fn load_plugin_skills(
             path,
             scope: SkillScope::User,
             file_system: Arc::clone(&LOCAL_FS),
+            plugin_id: Some(plugin_id.as_key()),
         })
         .collect::<Vec<_>>();
     let outcome = load_skills_from_roots(roots).await;