[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.schemas.json

@@ -627,6 +627,30 @@
           "title": "Skills/listRequest",
           "type": "object"
         },
+        {
+          "properties": {
+            "id": {
+              "$ref": "#/definitions/v2/RequestId"
+            },
+            "method": {
+              "enum": [
+                "marketplace/add"
+              ],
+              "title": "Marketplace/addRequestMethod",
+              "type": "string"
+            },
+            "params": {
+              "$ref": "#/definitions/v2/MarketplaceAddParams"
+            }
+          },
+          "required": [
+            "id",
+            "method",
+            "params"
+          ],
+          "title": "Marketplace/addRequest",
+          "type": "object"
+        },
         {
           "properties": {
             "id": {
@@ -9060,6 +9084,55 @@
         "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/v2/AbsolutePathBuf"
+          },
+          "marketplaceName": {
+            "type": "string"
+          }
+        },
+        "required": [
+          "alreadyAdded",
+          "installedRoot",
+          "marketplaceName"
+        ],
+        "title": "MarketplaceAddResponse",
+        "type": "object"
+      },
       "MarketplaceInterface": {
         "properties": {
           "displayName": {