Notify hooks should fire when user input is requested

When Codex enters an interactive prompt (e.g. multiple-choice) via request_user_input, no notify hook event is emitted, so external notifiers (HTTP/desktop/etc.) never trigger.

Expected

• A hook event should fire when Codex is waiting for user input, similar to after-tool-use.

Proposed fix

• Add a user-input-requested hook/notify event.
• Add notify_events config to opt-in which events trigger notify.

I have a working patch in my fork/branch and can open a PR if contributions are accepted (I saw the contribution policy mentions invitation-only PRs).