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-protocol/src/protocol/v2.rs

@@ -25,6 +25,7 @@ use codex_protocol::protocol::RateLimitSnapshot as CoreRateLimitSnapshot;
 use codex_protocol::protocol::RateLimitWindow as CoreRateLimitWindow;
 use codex_protocol::protocol::SessionSource as CoreSessionSource;
 use codex_protocol::protocol::SkillErrorInfo as CoreSkillErrorInfo;
+use codex_protocol::protocol::SkillInterface as CoreSkillInterface;
 use codex_protocol::protocol::SkillMetadata as CoreSkillMetadata;
 use codex_protocol::protocol::SkillScope as CoreSkillScope;
 use codex_protocol::protocol::TokenUsage as CoreTokenUsage;
@@ -1252,13 +1253,35 @@ pub enum SkillScope {
 pub struct SkillMetadata {
     pub name: String,
     pub description: String,
-    #[ts(optional)]
     #[serde(default, skip_serializing_if = "Option::is_none")]
+    #[ts(optional)]
+    /// Legacy short_description from SKILL.md. Prefer SKILL.toml interface.short_description.
     pub short_description: Option<String>,
+    #[serde(default, skip_serializing_if = "Option::is_none")]
+    #[ts(optional)]
+    pub interface: Option<SkillInterface>,
     pub path: PathBuf,
     pub scope: SkillScope,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct SkillInterface {
+    #[ts(optional)]
+    pub display_name: Option<String>,
+    #[ts(optional)]
+    pub short_description: Option<String>,
+    #[ts(optional)]
+    pub icon_small: Option<PathBuf>,
+    #[ts(optional)]
+    pub icon_large: Option<PathBuf>,
+    #[ts(optional)]
+    pub brand_color: Option<String>,
+    #[ts(optional)]
+    pub default_prompt: Option<String>,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
@@ -1282,12 +1305,26 @@ impl From<CoreSkillMetadata> for SkillMetadata {
             name: value.name,
             description: value.description,
             short_description: value.short_description,
+            interface: value.interface.map(SkillInterface::from),
             path: value.path,
             scope: value.scope.into(),
         }
     }
 }
 
+impl From<CoreSkillInterface> for SkillInterface {
+    fn from(value: CoreSkillInterface) -> Self {
+        Self {
+            display_name: value.display_name,
+            short_description: value.short_description,
+            brand_color: value.brand_color,
+            default_prompt: value.default_prompt,
+            icon_small: value.icon_small,
+            icon_large: value.icon_large,
+        }
+    }
+}
+
 impl From<CoreSkillScope> for SkillScope {
     fn from(value: CoreSkillScope) -> Self {
         match value {