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/schema/json/ClientRequest.json

@@ -2217,6 +2217,25 @@
       ],
       "type": "object"
     },
+    "PluginSkillReadParams": {
+      "properties": {
+        "remoteMarketplaceName": {
+          "type": "string"
+        },
+        "remotePluginId": {
+          "type": "string"
+        },
+        "skillName": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "remoteMarketplaceName",
+        "remotePluginId",
+        "skillName"
+      ],
+      "type": "object"
+    },
     "PluginUninstallParams": {
       "properties": {
         "pluginId": {
@@ -5036,6 +5055,30 @@
       "title": "Plugin/readRequest",
       "type": "object"
     },
+    {
+      "properties": {
+        "id": {
+          "$ref": "#/definitions/RequestId"
+        },
+        "method": {
+          "enum": [
+            "plugin/skill/read"
+          ],
+          "title": "Plugin/skill/readRequestMethod",
+          "type": "string"
+        },
+        "params": {
+          "$ref": "#/definitions/PluginSkillReadParams"
+        }
+      },
+      "required": [
+        "id",
+        "method",
+        "params"
+      ],
+      "title": "Plugin/skill/readRequest",
+      "type": "object"
+    },
     {
       "properties": {
         "id": {

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.schemas.json

@@ -762,6 +762,30 @@
           "title": "Plugin/readRequest",
           "type": "object"
         },
+        {
+          "properties": {
+            "id": {
+              "$ref": "#/definitions/v2/RequestId"
+            },
+            "method": {
+              "enum": [
+                "plugin/skill/read"
+              ],
+              "title": "Plugin/skill/readRequestMethod",
+              "type": "string"
+            },
+            "params": {
+              "$ref": "#/definitions/v2/PluginSkillReadParams"
+            }
+          },
+          "required": [
+            "id",
+            "method",
+            "params"
+          ],
+          "title": "Plugin/skill/readRequest",
+          "type": "object"
+        },
         {
           "properties": {
             "id": {
@@ -12391,6 +12415,40 @@
         "title": "PluginShareSaveResponse",
         "type": "object"
       },
+      "PluginSkillReadParams": {
+        "$schema": "http://json-schema.org/draft-07/schema#",
+        "properties": {
+          "remoteMarketplaceName": {
+            "type": "string"
+          },
+          "remotePluginId": {
+            "type": "string"
+          },
+          "skillName": {
+            "type": "string"
+          }
+        },
+        "required": [
+          "remoteMarketplaceName",
+          "remotePluginId",
+          "skillName"
+        ],
+        "title": "PluginSkillReadParams",
+        "type": "object"
+      },
+      "PluginSkillReadResponse": {
+        "$schema": "http://json-schema.org/draft-07/schema#",
+        "properties": {
+          "contents": {
+            "type": [
+              "string",
+              "null"
+            ]
+          }
+        },
+        "title": "PluginSkillReadResponse",
+        "type": "object"
+      },
       "PluginSource": {
         "oneOf": [
           {

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -1521,6 +1521,30 @@
           "title": "Plugin/readRequest",
           "type": "object"
         },
+        {
+          "properties": {
+            "id": {
+              "$ref": "#/definitions/RequestId"
+            },
+            "method": {
+              "enum": [
+                "plugin/skill/read"
+              ],
+              "title": "Plugin/skill/readRequestMethod",
+              "type": "string"
+            },
+            "params": {
+              "$ref": "#/definitions/PluginSkillReadParams"
+            }
+          },
+          "required": [
+            "id",
+            "method",
+            "params"
+          ],
+          "title": "Plugin/skill/readRequest",
+          "type": "object"
+        },
         {
           "properties": {
             "id": {
@@ -9044,6 +9068,40 @@
       "title": "PluginShareSaveResponse",
       "type": "object"
     },
+    "PluginSkillReadParams": {
+      "$schema": "http://json-schema.org/draft-07/schema#",
+      "properties": {
+        "remoteMarketplaceName": {
+          "type": "string"
+        },
+        "remotePluginId": {
+          "type": "string"
+        },
+        "skillName": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "remoteMarketplaceName",
+        "remotePluginId",
+        "skillName"
+      ],
+      "title": "PluginSkillReadParams",
+      "type": "object"
+    },
+    "PluginSkillReadResponse": {
+      "$schema": "http://json-schema.org/draft-07/schema#",
+      "properties": {
+        "contents": {
+          "type": [
+            "string",
+            "null"
+          ]
+        }
+      },
+      "title": "PluginSkillReadResponse",
+      "type": "object"
+    },
     "PluginSource": {
       "oneOf": [
         {

codex-rs/app-server-protocol/schema/json/v2/PluginSkillReadParams.json

@@ -0,0 +1,21 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "properties": {
+    "remoteMarketplaceName": {
+      "type": "string"
+    },
+    "remotePluginId": {
+      "type": "string"
+    },
+    "skillName": {
+      "type": "string"
+    }
+  },
+  "required": [
+    "remoteMarketplaceName",
+    "remotePluginId",
+    "skillName"
+  ],
+  "title": "PluginSkillReadParams",
+  "type": "object"
+}

codex-rs/app-server-protocol/schema/json/v2/PluginSkillReadResponse.json

@@ -0,0 +1,13 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "properties": {
+    "contents": {
+      "type": [
+        "string",
+        "null"
+      ]
+    }
+  },
+  "title": "PluginSkillReadResponse",
+  "type": "object"
+}

codex-rs/app-server-protocol/schema/typescript/v2/PluginSkillReadParams.ts

@@ -0,0 +1,5 @@
+// GENERATED CODE! DO NOT MODIFY BY HAND!
+
+// This file was generated by [ts-rs](https://github.com/Aleph-Alpha/ts-rs). Do not edit this file manually.
+
+export type PluginSkillReadParams = { remoteMarketplaceName: string, remotePluginId: string, skillName: string, };

codex-rs/app-server-protocol/schema/typescript/v2/PluginSkillReadResponse.ts

@@ -0,0 +1,5 @@
+// GENERATED CODE! DO NOT MODIFY BY HAND!
+
+// This file was generated by [ts-rs](https://github.com/Aleph-Alpha/ts-rs). Do not edit this file manually.
+
+export type PluginSkillReadResponse = { contents: string | null, };

codex-rs/app-server-protocol/schema/typescript/v2/index.ts

@@ -287,6 +287,8 @@ export type { PluginShareListParams } from "./PluginShareListParams";
 export type { PluginShareListResponse } from "./PluginShareListResponse";
 export type { PluginShareSaveParams } from "./PluginShareSaveParams";
 export type { PluginShareSaveResponse } from "./PluginShareSaveResponse";
+export type { PluginSkillReadParams } from "./PluginSkillReadParams";
+export type { PluginSkillReadResponse } from "./PluginSkillReadResponse";
 export type { PluginSource } from "./PluginSource";
 export type { PluginSummary } from "./PluginSummary";
 export type { PluginUninstallParams } from "./PluginUninstallParams";

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

@@ -612,6 +612,11 @@ client_request_definitions! {
         serialization: global("config"),
         response: v2::PluginReadResponse,
     },
+    PluginSkillRead => "plugin/skill/read" {
+        params: v2::PluginSkillReadParams,
+        serialization: global("config"),
+        response: v2::PluginSkillReadResponse,
+    },
     PluginShareSave => "plugin/share/save" {
         params: v2::PluginShareSaveParams,
         serialization: global("config"),

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) {