Preserve discovery order for host skill path aliases (#36327)

## Why

Host skill path aliases should follow the core skill loader's root discovery
order, independent of how catalog entries are sorted for rendering.

## What changed

- Propagate each host skill root's discovery position into its catalog entries.
- Assign `r0`, `r1`, and subsequent aliases from that position while leaving
  entries without an explicit root order last.

## Testing

Added coverage for alias ordering in extension rendering and across consecutive
production turns.

GitOrigin-RevId: 9085a3843c91bf2ce78a378cf5ce4926e9c38eb8

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

@@ -61,6 +61,11 @@ impl SkillLoadOutcome {
         self.skill_root_by_path.get(path)
     }
 
+    /// Returns loaded skill roots in discovery order.
+    pub fn skill_roots_in_discovery_order(&self) -> impl Iterator<Item = &AbsolutePathBuf> {
+        self.skill_roots.iter()
+    }
+
     pub(crate) fn file_system_for_skill(
         &self,
         skill: &SkillMetadata,