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/schema/json/v2/PluginShareCheckoutParams.json

@@ -0,0 +1,13 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "properties": {
+    "remotePluginId": {
+      "type": "string"
+    }
+  },
+  "required": [
+    "remotePluginId"
+  ],
+  "title": "PluginShareCheckoutParams",
+  "type": "object"
+}

codex-rs/app-server-protocol/schema/json/v2/PluginShareCheckoutResponse.json

@@ -0,0 +1,45 @@
+{
+  "$schema": "http://json-schema.org/draft-07/schema#",
+  "definitions": {
+    "AbsolutePathBuf": {
+      "description": "A path that is guaranteed to be absolute and normalized (though it is not guaranteed to be canonicalized or exist on the filesystem).\n\nIMPORTANT: When deserializing an `AbsolutePathBuf`, a base path must be set using [AbsolutePathBufGuard::new]. If no base path is set, the deserialization will fail unless the path being deserialized is already absolute.",
+      "type": "string"
+    }
+  },
+  "properties": {
+    "marketplaceName": {
+      "type": "string"
+    },
+    "marketplacePath": {
+      "$ref": "#/definitions/AbsolutePathBuf"
+    },
+    "pluginId": {
+      "type": "string"
+    },
+    "pluginName": {
+      "type": "string"
+    },
+    "pluginPath": {
+      "$ref": "#/definitions/AbsolutePathBuf"
+    },
+    "remotePluginId": {
+      "type": "string"
+    },
+    "remoteVersion": {
+      "type": [
+        "string",
+        "null"
+      ]
+    }
+  },
+  "required": [
+    "marketplaceName",
+    "marketplacePath",
+    "pluginId",
+    "pluginName",
+    "pluginPath",
+    "remotePluginId"
+  ],
+  "title": "PluginShareCheckoutResponse",
+  "type": "object"
+}