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/ext/skills/tests/skills_extension.rs

@@ -175,7 +175,6 @@ async fn installed_extension_uses_host_service_snapshot() -> TestResult {
         name: "demo".to_string(),
         description: "Demo skill.".to_string(),
         short_description: None,
-        model: None,
         interface: None,
         dependencies: None,
         policy: None,
@@ -257,7 +256,6 @@ async fn host_world_state_records_catalog_metrics_on_publish_and_change() -> Tes
         name: "demo".to_string(),
         description: "Demo skill.".to_string(),
         short_description: None,
-        model: None,
         interface: None,
         dependencies: None,
         policy: None,
@@ -322,7 +320,6 @@ async fn host_world_state_records_catalog_metrics_on_publish_and_change() -> Tes
         name: "other".to_string(),
         description: "Other skill.".to_string(),
         short_description: None,
-        model: None,
         interface: None,
         dependencies: None,
         policy: None,
@@ -375,7 +372,6 @@ async fn persisted_host_snapshot_deduplicates_warning_after_reinitialization() -
         name: "demo".to_string(),
         description: "Demo skill.".to_string(),
         short_description: None,
-        model: None,
         interface: None,
         dependencies: None,
         policy: None,