Support enable/disable skill via config/api. (#9328)

In config.toml:
```
[[skills.config]]
path = "/Users/xl/.codex/skills/my_skill/SKILL.md"
enabled = false
```

API:
skills/list, skills/config/write

codex-rs/app-server-protocol/src/protocol/common.rs

@@ -133,6 +133,10 @@ client_request_definitions! {
         params: v2::SkillsListParams,
         response: v2::SkillsListResponse,
     },
+    SkillsConfigWrite => "skills/config/write" {
+        params: v2::SkillsConfigWriteParams,
+        response: v2::SkillsConfigWriteResponse,
+    },
     TurnStart => "turn/start" {
         params: v2::TurnStartParams,
         response: v2::TurnStartResponse,

codex-rs/app-server-protocol/src/protocol/v2.rs

@@ -1274,6 +1274,7 @@ pub struct SkillMetadata {
     pub interface: Option<SkillInterface>,
     pub path: PathBuf,
     pub scope: SkillScope,
+    pub enabled: bool,
 }
 
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
@@ -1311,6 +1312,21 @@ pub struct SkillsListEntry {
     pub errors: Vec<SkillErrorInfo>,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct SkillsConfigWriteParams {
+    pub path: PathBuf,
+    pub enabled: bool,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct SkillsConfigWriteResponse {
+    pub effective_enabled: bool,
+}
+
 impl From<CoreSkillMetadata> for SkillMetadata {
     fn from(value: CoreSkillMetadata) -> Self {
         Self {
@@ -1320,6 +1336,7 @@ impl From<CoreSkillMetadata> for SkillMetadata {
             interface: value.interface.map(SkillInterface::from),
             path: value.path,
             scope: value.scope.into(),
+            enabled: true,
         }
     }
 }