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/codex_app_server_protocol.schemas.json

@@ -877,6 +877,30 @@
           "title": "Plugin/share/listRequest",
           "type": "object"
         },
+        {
+          "properties": {
+            "id": {
+              "$ref": "#/definitions/v2/RequestId"
+            },
+            "method": {
+              "enum": [
+                "plugin/share/checkout"
+              ],
+              "title": "Plugin/share/checkoutRequestMethod",
+              "type": "string"
+            },
+            "params": {
+              "$ref": "#/definitions/v2/PluginShareCheckoutParams"
+            }
+          },
+          "required": [
+            "id",
+            "method",
+            "params"
+          ],
+          "title": "Plugin/share/checkoutRequest",
+          "type": "object"
+        },
         {
           "properties": {
             "id": {
@@ -12268,6 +12292,58 @@
         "title": "PluginReadResponse",
         "type": "object"
       },
+      "PluginShareCheckoutParams": {
+        "$schema": "http://json-schema.org/draft-07/schema#",
+        "properties": {
+          "remotePluginId": {
+            "type": "string"
+          }
+        },
+        "required": [
+          "remotePluginId"
+        ],
+        "title": "PluginShareCheckoutParams",
+        "type": "object"
+      },
+      "PluginShareCheckoutResponse": {
+        "$schema": "http://json-schema.org/draft-07/schema#",
+        "properties": {
+          "marketplaceName": {
+            "type": "string"
+          },
+          "marketplacePath": {
+            "$ref": "#/definitions/v2/AbsolutePathBuf"
+          },
+          "pluginId": {
+            "type": "string"
+          },
+          "pluginName": {
+            "type": "string"
+          },
+          "pluginPath": {
+            "$ref": "#/definitions/v2/AbsolutePathBuf"
+          },
+          "remotePluginId": {
+            "type": "string"
+          },
+          "remoteVersion": {
+            "type": [
+              "string",
+              "null"
+            ]
+          }
+        },
+        "required": [
+          "marketplaceName",
+          "marketplacePath",
+          "pluginId",
+          "pluginName",
+          "pluginPath",
+          "remotePluginId"
+        ],
+        "title": "PluginShareCheckoutResponse",
+        "type": "object"
+      },
       "PluginShareContext": {
         "properties": {
           "creatorAccountUserId": {