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/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": [
         {