Remove skill model delegation support (#39068)

## What changed

- Stop parsing and exposing the `model` field from skill frontmatter.
- Remove the skill model delegation types and instruction generation.

GitOrigin-RevId: 01198c68c095da5062e8abec417cc3539099d9d5

codex-rs/skills/src/selection_tests.rs

@@ -32,7 +32,6 @@ fn make_skill(name: &str, path: &str) -> SkillMetadata {
         name: name.to_string(),
         description: format!("{name} skill"),
         short_description: None,
-        model: None,
         interface: None,
         dependencies: None,
         policy: None,