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/app-server/src/codex_message_processor.rs

@@ -3894,6 +3894,16 @@ fn skills_to_info(
             name: skill.name.clone(),
             description: skill.description.clone(),
             short_description: skill.short_description.clone(),
+            interface: skill.interface.clone().map(|interface| {
+                codex_app_server_protocol::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.into(),
         })