Strengthen sync-models prompt to prevent adding undocumented models

[Copilot]

The automated model sync was adding models (e.g. qwen2.5) that don't appear on the GitHub Copilot supported models docs page, likely sourced from the model's own training knowledge.

Changes to sync-models-prompt.md

• Requirements section: Added a prominent > IMPORTANT blockquote immediately after the fetch/compare steps, explicitly forbidding models from any source other than the docs page
• Metadata Updates section: Replaced the soft "do not make up model names" note with a clearer directive tied to the docs page
• Constraints section: Added a bold first bullet that restates the restriction with the full docs URL, covering sources like training knowledge, third-party lists, or speculation