Load skill interface metadata in the host loader (#36903)

## What changed

- Move skill interface validation and asset-path resolution into `codex-skills` so both skill loaders share the same behavior.
- Populate host-loaded skills with validated display metadata, prompts, brand colors, and local icons from `agents/openai.yaml`.
- Allow plugin skills to reference icons under the plugin's shared `assets` directory while rejecting absolute paths and paths that escape permitted asset roots.

## Testing

- Add resolver tests for valid, invalid, local, and plugin-shared interface assets.
- Add host loader coverage for interface fields and asset-path restrictions.

GitOrigin-RevId: 3f316191752eeebd15d11ac0bee82201574320ec

codex-rs/skills/src/lib.rs

@@ -1,6 +1,10 @@
+mod interface;
 mod model;
 mod parser;
 
+pub use interface::SkillInterfaceAssetPolicy;
+pub use interface::SkillInterfaceFile;
+pub use interface::resolve_skill_interface;
 pub use model::EnvironmentSkillMetadata;
 pub use model::SkillConfigRule;
 pub use model::SkillConfigRuleSelector;