Add remote plugin skill read API (#20150)

## Summary

Adds an app-server `plugin/skill/read` method for remote plugin skill
markdown. The new method calls the plugin-service skill detail endpoint
and returns `skill_md_contents`, so clients can preview skills for
remote plugins before the bundle is installed locally.

## Why

Uninstalled remote plugin skills do not have local `SKILL.md` files.
Without an on-demand remote read, the desktop plugin details UI cannot
render the skill details modal for those skills.

## Validation

- `just write-app-server-schema`
- `just fmt`
- `cargo test -p codex-app-server-protocol`
- `cargo test -p codex-app-server --test all --
suite::v2::plugin_read::plugin_skill_read_reads_remote_skill_contents_when_remote_plugin_enabled
--exact`
- `just fix -p codex-app-server-protocol -p codex-core-plugins -p
codex-app-server`

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

@@ -4609,6 +4609,22 @@ pub struct PluginReadResponse {
     pub plugin: PluginDetail,
 }
 
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct PluginSkillReadParams {
+    pub remote_marketplace_name: String,
+    pub remote_plugin_id: String,
+    pub skill_name: String,
+}
+
+#[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
+#[serde(rename_all = "camelCase")]
+#[ts(export_to = "v2/")]
+pub struct PluginSkillReadResponse {
+    pub contents: Option<String>,
+}
+
 #[derive(Serialize, Deserialize, Debug, Clone, PartialEq, JsonSchema, TS)]
 #[serde(rename_all = "camelCase")]
 #[ts(export_to = "v2/")]
@@ -10667,6 +10683,23 @@ mod tests {
         );
     }
 
+    #[test]
+    fn plugin_skill_read_params_serialization_uses_remote_plugin_id() {
+        assert_eq!(
+            serde_json::to_value(PluginSkillReadParams {
+                remote_marketplace_name: "chatgpt-global".to_string(),
+                remote_plugin_id: "plugins~Plugin_00000000000000000000000000000000".to_string(),
+                skill_name: "plan-work".to_string(),
+            })
+            .unwrap(),
+            json!({
+                "remoteMarketplaceName": "chatgpt-global",
+                "remotePluginId": "plugins~Plugin_00000000000000000000000000000000",
+                "skillName": "plan-work",
+            }),
+        );
+    }
+
     #[test]
     fn plugin_share_params_and_response_serialization_use_camel_case_fields() {
         let plugin_path = if cfg!(windows) {