Load plugin skill roots through the host skills service (#37440)

## What changed

- Implement `SkillRootLoader<PluginSkillRoot>` for `HostSkillsService`.
- Load roots concurrently while preserving request order, product restrictions, cached snapshots, skill errors, and per-skill filesystem metadata.
- Deduplicate overlapping roots and prefer native plugin skills over migrated commands with the same plugin ID and skill name.

## Testing

- Cover native and migrated skill precedence, nested and overlapping roots, product filtering, snapshot reuse, error propagation, and symlinked migration roots.

GitOrigin-RevId: 2270d10616b3a0463852c563ee071c524cd11e6b

codex-rs/core-skills/src/model.rs

@@ -31,6 +31,28 @@ pub struct SkillLoadOutcome {
 }
 
 impl SkillLoadOutcome {
+    /// Builds an already-composed outcome while retaining the filesystem that supplied each skill.
+    pub fn from_parts(
+        skills: Vec<SkillMetadata>,
+        errors: Vec<SkillError>,
+        skill_roots: Vec<AbsolutePathBuf>,
+        skill_root_by_path: HashMap<AbsolutePathBuf, AbsolutePathBuf>,
+        skill_discovery_path_by_path: HashMap<AbsolutePathBuf, AbsolutePathBuf>,
+        agent_plugin_skill_paths: HashSet<AbsolutePathBuf>,
+        file_systems_by_skill_path: HashMap<AbsolutePathBuf, Arc<dyn ExecutorFileSystem>>,
+    ) -> Self {
+        Self {
+            skills,
+            errors,
+            skill_roots,
+            skill_root_by_path: Arc::new(skill_root_by_path),
+            skill_discovery_path_by_path: Arc::new(skill_discovery_path_by_path),
+            agent_plugin_skill_paths,
+            file_systems_by_skill_path: SkillFileSystemsByPath::new(file_systems_by_skill_path),
+            ..Self::default()
+        }
+    }
+
     pub fn is_skill_enabled(&self, skill: &SkillMetadata) -> bool {
         !self.disabled_paths.contains(&skill.path_to_skills_md)
     }