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/tui/src/chatwidget.rs

@@ -87,6 +87,7 @@ use codex_core::protocol::ViewImageToolCallEvent;
 use codex_core::protocol::WarningEvent;
 use codex_core::protocol::WebSearchBeginEvent;
 use codex_core::protocol::WebSearchEndEvent;
+use codex_core::skills::model::SkillInterface;
 use codex_core::skills::model::SkillMetadata;
 use codex_protocol::ThreadId;
 use codex_protocol::account::PlanType;
@@ -4594,6 +4595,14 @@ fn skills_for_cwd(cwd: &Path, skills_entries: &[SkillsListEntry]) -> Vec<SkillMe
                     name: skill.name.clone(),
                     description: skill.description.clone(),
                     short_description: skill.short_description.clone(),
+                    interface: skill.interface.clone().map(|interface| SkillInterface {
+                        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,
                 })