[codex] Refactor marketplace add into shared core flow (#17717)

## Summary

Move `codex marketplace add` onto a shared core implementation so the
CLI and app-server path can use one source of truth.

This change:
- adds shared marketplace-add orchestration in `codex-core`
- switches the CLI command to call that shared implementation
- removes duplicated CLI-only marketplace add helpers
- preserves focused parser and add-path coverage while moving the shared
behavior into core tests

## Why

The new `marketplace/add` RPC should reuse the same underlying
marketplace-add flow as the CLI. This refactor lands that consolidation
first so the follow-up app-server PR can be mostly protocol and handler
wiring.

## Validation

- `cargo test -p codex-core marketplace_add`
- `cargo test -p codex-cli marketplace_cmd`
- `just fix -p codex-core`
- `just fix -p codex-cli`
- `just fmt`

codex-rs/app-server-protocol/schema/json/codex_app_server_protocol.v2.schemas.json

@@ -1209,6 +1209,30 @@
           "title": "Skills/listRequest",
           "type": "object"
         },
+        {
+          "properties": {
+            "id": {
+              "$ref": "#/definitions/RequestId"
+            },
+            "method": {
+              "enum": [
+                "marketplace/add"
+              ],
+              "title": "Marketplace/addRequestMethod",
+              "type": "string"
+            },
+            "params": {
+              "$ref": "#/definitions/MarketplaceAddParams"
+            }
+          },
+          "required": [
+            "id",
+            "method",
+            "params"
+          ],
+          "title": "Marketplace/addRequest",
+          "type": "object"
+        },
         {
           "properties": {
             "id": {
@@ -5856,6 +5880,55 @@
       "title": "LogoutAccountResponse",
       "type": "object"
     },
+    "MarketplaceAddParams": {
+      "$schema": "http://json-schema.org/draft-07/schema#",
+      "properties": {
+        "refName": {
+          "type": [
+            "string",
+            "null"
+          ]
+        },
+        "source": {
+          "type": "string"
+        },
+        "sparsePaths": {
+          "items": {
+            "type": "string"
+          },
+          "type": [
+            "array",
+            "null"
+          ]
+        }
+      },
+      "required": [
+        "source"
+      ],
+      "title": "MarketplaceAddParams",
+      "type": "object"
+    },
+    "MarketplaceAddResponse": {
+      "$schema": "http://json-schema.org/draft-07/schema#",
+      "properties": {
+        "alreadyAdded": {
+          "type": "boolean"
+        },
+        "installedRoot": {
+          "$ref": "#/definitions/AbsolutePathBuf"
+        },
+        "marketplaceName": {
+          "type": "string"
+        }
+      },
+      "required": [
+        "alreadyAdded",
+        "installedRoot",
+        "marketplaceName"
+      ],
+      "title": "MarketplaceAddResponse",
+      "type": "object"
+    },
     "MarketplaceInterface": {
       "properties": {
         "displayName": {