Add turn started/completed events and correct exit code on error (#4309)

Adds new event for session completed that includes usage. Also ensures
we return 1 on failures.
```
{
  "type": "session.created",
  "session_id": "019987a7-93e7-7b20-9e05-e90060e411ea"
}
{
  "type": "turn.started"
}
...
{
  "type": "turn.completed",
  "usage": {
    "input_tokens": 78913,
    "cached_input_tokens": 65280,
    "output_tokens": 1099
  }
}
```

README.md

@@ -1,4 +1,3 @@
-<h1 align="center">OpenAI Codex CLI</h1>
 
 <p align="center"><code>npm i -g @openai/codex</code><br />or <code>brew install codex</code></p>
 
@@ -102,4 +101,3 @@ Codex CLI supports a rich set of configuration options, with preferences stored
 ## License
 
 This repository is licensed under the [Apache-2.0 License](LICENSE).
-