docs: clarify auth in Codex CLI instructions

Author: RossTarrant

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
 ```
 
+The `--bearer-token-env-var` option is required for PAT-authenticated access to the hosted GitHub MCP server.
+
 <details>
 <summary><b>Storing Your PAT Securely</b></summary>
 <br>