## What changed
- Add `StartThreadOptions::new` to provide the standard configuration for a new thread.
- Make `ThreadManager::start_thread` the single thread-start entry point and migrate callers from the previous convenience methods.
- Derive default environment selections when `environments` is `None`, while preserving explicit selections, including an empty list.
GitOrigin-RevId: 8977dc11aed54c5e1215a81eaed2b2cf5fc6087a
## What changed
- Add `codex-agent-extension` with an `AgentRunner` that starts a resolved agent prompt in a thread forked from its parent.
- Propagate the invocation's trace context, select the configured execution environments, submit the initial prompt, and return the spawned thread and turn identifiers.
- Reject empty agent prompts and report when the owning thread manager is no longer available.
## Testing
- Add an integration test that verifies the agent runs in a forked thread, returns the started turn identifier, completes the turn, and sends the resolved prompt to the model.
GitOrigin-RevId: ffb805efabadf758969d6611a6867db6fa9f059a
