Add UI for skill enable/disable. (#9627)

"/skill" will now allow you to enable/disable skills:
<img width="658" height="199" alt="image"
src="https://github.com/user-attachments/assets/bf8994c8-d6c1-462f-8bbb-f1ee9241caa4"
/>

codex-rs/protocol/src/protocol.rs

@@ -2058,6 +2058,7 @@ pub struct SkillMetadata {
     pub interface: Option<SkillInterface>,
     pub path: PathBuf,
     pub scope: SkillScope,
+    pub enabled: bool,
 }
 
 #[derive(Debug, Clone, Deserialize, Serialize, JsonSchema, TS, PartialEq, Eq)]