feat: Compress skill paths with root aliases (#19098)

Add skill root tracking so model-visible skill lists can use short path
aliases when absolute paths would exceed the metadata budget.

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

@@ -159,13 +159,22 @@ where
     I: IntoIterator<Item = SkillRoot>,
 {
     let mut outcome = SkillLoadOutcome::default();
+    let mut skill_roots: Vec<AbsolutePathBuf> = Vec::new();
+    let mut skill_root_by_path: HashMap<AbsolutePathBuf, AbsolutePathBuf> = HashMap::new();
     let mut file_systems_by_skill_path: HashMap<AbsolutePathBuf, Arc<dyn ExecutorFileSystem>> =
         HashMap::new();
     for root in roots {
+        let root_path = canonicalize_for_skill_identity(&root.path);
         let fs = root.file_system;
         let skills_before_root = outcome.skills.len();
-        discover_skills_under_root(fs.as_ref(), &root.path, root.scope, &mut outcome).await;
+        discover_skills_under_root(fs.as_ref(), &root_path, root.scope, &mut outcome).await;
         for skill in &outcome.skills[skills_before_root..] {
+            if !skill_roots.contains(&root_path) {
+                skill_roots.push(root_path.clone());
+            }
+            skill_root_by_path
+                .entry(skill.path_to_skills_md.clone())
+                .or_insert_with(|| root_path.clone());
             file_systems_by_skill_path
                 .entry(skill.path_to_skills_md.clone())
                 .or_insert_with(|| Arc::clone(&fs));
@@ -181,7 +190,12 @@ where
         .iter()
         .map(|skill| skill.path_to_skills_md.clone())
         .collect();
+    skill_root_by_path.retain(|path, _| retained_skill_paths.contains(path));
+    let used_roots: HashSet<AbsolutePathBuf> = skill_root_by_path.values().cloned().collect();
+    skill_roots.retain(|root| used_roots.contains(root));
     file_systems_by_skill_path.retain(|path, _| retained_skill_paths.contains(path));
+    outcome.skill_roots = skill_roots;
+    outcome.skill_root_by_path = Arc::new(skill_root_by_path);
     outcome.file_systems_by_skill_path = SkillFileSystemsByPath::new(file_systems_by_skill_path);
 
     fn scope_rank(scope: SkillScope) -> u8 {