docs(codex): include PAT binding in install guide

Author: 20bytes

docs/installation-guides/install-codex.md

@@ -21,9 +21,11 @@ bearer_token_env_var = "GITHUB_PAT_TOKEN"
 You can also add it via the Codex CLI:
 
 ```cli
-codex mcp add github --url https://api.githubcopilot.com/mcp/  
+codex mcp add github --url https://api.githubcopilot.com/mcp/ --bearer-token-env-var GITHUB_PAT_TOKEN
 ```
 
+Using `codex mcp add ... --url ...` without `--bearer-token-env-var` only stores the remote server URL. For the hosted GitHub MCP server, you also need to bind a PAT environment variable so Codex can send the `Authorization` header.
+
 <details>
 <summary><b>Storing Your PAT Securely</b></summary>
 <br>