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/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"),