Read executor skill packages directly (#38167)

## Why

Executor skills required a `skills.list` lookup before their contents could be
read, even when the catalog already identified the skill.

## What changed

- Render executor skill catalog entries as package locators, including shortened
  locators when skill roots are aliased.
- Tell the model to pass those packages directly to `skills.read`, while keeping
  `skills.list` as the fallback when no package is available.
- Use the shorter package locators when applying the skills context budget.

## Testing

Update catalog, aliasing, budget-pressure, and selected-skill coverage, including
an executor package ID containing a literal backslash.

GitOrigin-RevId: 2ace5e527dc4f29651a8f3214475ccc90d52c73f

codex-rs/app-server/tests/suite/v2/selected_capability_stack.rs

@@ -608,7 +608,7 @@ fn assert_selected_skill_catalog_available(request: &ResponsesRequest) {
     let catalog_fragment = latest_selected_skill_update(request)
         .expect("selected skill catalog update should be model-visible");
     assert!(catalog_fragment.contains(SKILL_DESCRIPTION));
-    assert!(catalog_fragment.contains("environment resource:"));
+    assert!(catalog_fragment.contains("executor package:"));
 }
 
 fn latest_selected_skill_update(request: &ResponsesRequest) -> Option<String> {