## 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`
