fix(codex): add canonical marketplace manifest and correct install docs

The previous Codex install flow was broken on two levels:

1. The README told users to run `codex plugin install dodopayments@dodopayments`,
   which is not a real Codex subcommand (Codex CLI 0.129+ only supports
   `codex plugin marketplace {add,upgrade,remove}`; plugin installation
   happens via the `/plugins` slash command inside the TUI).
2. After `codex plugin marketplace add` succeeded, the plugin still did not
   appear under `/plugins` because the repo only shipped a Claude-Code
   marketplace at `.claude-plugin/marketplace.json`. Codex prefers
   `.agents/plugins/marketplace.json` with its own schema (object `source`,
   required `policy` and `category` fields). The Claude-Code-shaped manifest
   was not a valid alternate layout for our split (`.codex-plugin/plugin.json`).

Add `.agents/plugins/marketplace.json` pointing the `dodopayments` plugin at
the repo root (`./`), where `.codex-plugin/plugin.json` already lives, and
rewrite the README's Codex section to reflect the actual two-step flow.

Closes #4

Author: aagarwal1012

.agents/plugins/marketplace.json

@@ -0,0 +1,20 @@
+{
+    "name": "dodopayments",
+    "interface": {
+        "displayName": "Dodo Payments"
+    },
+    "plugins": [
+        {
+            "name": "dodopayments",
+            "source": {
+                "source": "local",
+                "path": "./"
+            },
+            "policy": {
+                "installation": "AVAILABLE",
+                "authentication": "ON_INSTALL"
+            },
+            "category": "Developer Tools"
+        }
+    ]
+}

README.md

@@ -26,13 +26,30 @@ The API MCP server uses browser OAuth by default, so no keys are required at ins
 
 ### Codex CLI
 
-Codex reads `.claude-plugin/marketplace.json` natively, so the same repo works:
+Codex installs plugins in two steps: register the marketplace from your shell, then install the plugin from inside the Codex TUI.
+
+1. Register the marketplace:
+
+    ```bash
+    codex plugin marketplace add dodopayments/dodo-agent-plugin
+    ```
+
+2. Open Codex and run the `/plugins` slash command:
+
+    ```bash
+    codex
+    ```
+
+    Then type `/plugins`, switch to the **Dodo Payments** marketplace, select the **dodopayments** plugin, and choose **Install plugin**.
+
+If you previously added the marketplace before this fix landed and the plugin doesn't appear under `/plugins`, refresh it:
 
 ```bash
-codex plugin marketplace add dodopayments/dodo-agent-plugin
-codex plugin install dodopayments@dodopayments
+codex plugin marketplace upgrade dodopayments
 ```
 
+> Codex CLI does not have a `codex plugin install` subcommand. Plugin installation always happens through the in-TUI `/plugins` flow ([official docs](https://developers.openai.com/codex/plugins)).
+
 ### Cursor
 
 Manual install: