[Feature]: Add Fast mode selection for ChatGPT

Summary

Let users choose Standard or Fast mode with the openai-chatgpt provider.

Motivation

OpenWiki does not expose the service-tier choice available in Codex. This option would let users spend more ChatGPT credits for faster responses during wiki generation.

Proposed solution

• Let users enable or disable Fast mode.
• Send service_tier: "priority" for Fast and "default" for Standard.
• Keep the current behavior when no choice is configured.
• Explain the higher ChatGPT usage when users select Fast.

The setup selector and /effort command from #634 provide a reference for the interface. I propose a similar setup option and a /fast command.

I would like to contribute this feature. Does this scope fit the project?