notify: emit agent-turn-start hook

[taf2]

External integrations (like lights) need a start signal, not just completion.

• Add a new notify payload type agent-turn-start so scripts can react when a turn begins.
• Emit it at the start of run_turn with thread/turn/cwd and user input messages.
• Update docs and tests to cover the new event.

[github-actions]

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[etraut-openai]

Our contribution guidelines indicate that we're not currently accepting contributions for new features at this time. We need to make sure that all new features compose well with both existing and upcoming features, fit into our roadmap, and are consistent across all Codex surfaces (CLI, IDE extension, web). If you would like to propose a new feature, please file or upvote an enhancement request in the issue tracker. We will generally prioritize new features based on community feedback.

We're working on designing a comprehensive "hooks" feature, so this will be coming to Codex soon.

[taf2]

@easong-openai no worries - this is all codex anyway 👍 looking forward to hooks feature will be super fun thanks