docs: update codex installation instructions (#135)

Author: twishabansal

README.md

@@ -173,50 +173,26 @@ _(Tip: Run `/plugin list` inside Claude Code to verify the plugin is active, or
 <details>
 <summary id="codex">Codex</summary>
 
-**1. Clone the Repo:**
+**1. Install marketplace:**
 
 ```bash
-git clone --branch 0.2.1 git@github.com:gemini-cli-extensions/bigquery-data-analytics.git
+codex plugin marketplace add GoogleCloudPlatform/data-agent-kit
 ```
 
 **2. Install the plugin:**
 
 ```bash
-mkdir -p ~/.codex/plugins
-cp -R /absolute/path/to/bigquery-data-analytics ~/.codex/plugins/bigquery-data-analytics
+codex plugin install bigquery-data-analytics@data-agent-kit
 ```
 
 **3. Set env vars:**
 Enter your environment vars as described in the [configuration section](#configuration).
 
-**4. Create or update marketplace.json:**
-`~/.agents/plugins/marketplace.json`
-
-```json
-{
-  "name": "my-data-cloud-google-marketplace",
-  "interface": {
-    "displayName": "Google Data Cloud Skills"
-  },
-  "plugins": [
-    {
-      "name": "bigquery-data-analytics",
-      "source": {
-        "source": "local",
-        "path": "./plugins/bigquery-data-analytics"
-      },
-      "policy": {
-        "installation": "AVAILABLE",
-        "authentication": "ON_INSTALL"
-      },
-      "category": "Database"
-    }
-  ]
-}
+**4. (Optional) Update the marketplace:**
+```sh
+codex plugin marketplace upgrade data-agent-kit
 ```
 
-_(Tip: Run `codex plugin list` or use the `/plugins` interactive menu to verify your installed plugins.)_
-
 </details>
 
 ## Installing using [open agent skills tool](https://github.com/vercel-labs/skills)