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/app-server-protocol/src/protocol/common.rs

@@ -649,6 +649,11 @@ client_request_definitions! {
         serialization: global("config"),
         response: v2::PluginShareListResponse,
     },
+    PluginShareCheckout => "plugin/share/checkout" {
+        params: v2::PluginShareCheckoutParams,
+        serialization: global("config"),
+        response: v2::PluginShareCheckoutResponse,
+    },
     PluginShareDelete => "plugin/share/delete" {
         params: v2::PluginShareDeleteParams,
         serialization: global("config"),