feat(extensions): add OpenAI Codex CLI auth provider

Adds a new authentication provider that reads OAuth tokens from the
OpenAI Codex CLI (~/.codex/auth.json) to authenticate with OpenAI's API.

This allows ChatGPT Plus/Pro subscribers to use OpenAI models in OpenClaw
without needing a separate API key - just authenticate with 'codex login'
first, then enable this plugin.

Features:
- Reads existing Codex CLI credentials from ~/.codex/auth.json
- Supports all Codex-available models (gpt-4.1, gpt-4o, o1, o3, etc.)
- Automatic token expiry detection from JWT
- Clear setup instructions and troubleshooting docs

Usage:
  openclaw plugins enable openai-codex-auth
  openclaw models auth login --provider openai-codex --set-default

extensions/openai-codex-auth/openclaw.plugin.json

@@ -0,0 +1,9 @@
+{
+  "id": "openai-codex-auth",
+  "providers": ["openai-codex"],
+  "configSchema": {
+    "type": "object",
+    "additionalProperties": false,
+    "properties": {}
+  }
+}