feat: Add plugin share checkout (#22435)

Adds plugin/share/checkout to turn a shared remote plugin into a local
working copy under ~/plugins/<name>.

Registers the copy in the managed personal marketplace and records the
remote-to-local mapping for later share/save flows.

---------

Co-authored-by: Codex <noreply@openai.com>

codex-rs/core-plugins/src/remote/share.rs

@@ -16,10 +16,13 @@ use std::io::Write;
 use std::path::Path;
 use tracing::warn;
 
+mod checkout;
 mod local_paths;
 
 const REMOTE_PLUGIN_SHARE_MAX_ARCHIVE_BYTES: usize = 50 * 1024 * 1024;
 
+pub use checkout::checkout_remote_plugin_share;
+
 #[derive(Debug, Clone, PartialEq, Eq)]
 pub struct RemotePluginShareSaveResult {
     pub remote_plugin_id: String,