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