Support SKILL.toml file. (#9125)

We’re introducing a new SKILL.toml to hold skill metadata so Codex can
deliver a richer Skills experience.

Initial focus is the interface block:
```
[interface]
display_name = "Optional user-facing name"
short_description = "Optional user-facing description"
icon_small = "./assets/small-400px.png"
icon_large = "./assets/large-logo.svg"
brand_color = "#3B82F6"
default_prompt = "Optional surrounding prompt to use the skill with"
```

All fields are exposed via the app server API.
display_name and short_description are consumed by the TUI.

codex-rs/core/src/codex.rs

@@ -120,6 +120,7 @@ use crate::protocol::ReviewDecision;
 use crate::protocol::SandboxPolicy;
 use crate::protocol::SessionConfiguredEvent;
 use crate::protocol::SkillErrorInfo;
+use crate::protocol::SkillInterface as ProtocolSkillInterface;
 use crate::protocol::SkillMetadata as ProtocolSkillMetadata;
 use crate::protocol::StreamErrorEvent;
 use crate::protocol::Submission;
@@ -2480,6 +2481,17 @@ fn skills_to_info(skills: &[SkillMetadata]) -> Vec<ProtocolSkillMetadata> {
             name: skill.name.clone(),
             description: skill.description.clone(),
             short_description: skill.short_description.clone(),
+            interface: skill
+                .interface
+                .clone()
+                .map(|interface| ProtocolSkillInterface {
+                    display_name: interface.display_name,
+                    short_description: interface.short_description,
+                    icon_small: interface.icon_small,
+                    icon_large: interface.icon_large,
+                    brand_color: interface.brand_color,
+                    default_prompt: interface.default_prompt,
+                }),
             path: skill.path.clone(),
             scope: skill.scope,
         })