[DevTask] Migrate .cursorrules to Project Rules

`.cursorrules` is deprecated, this issue proposes a move to [Cursor's project rules](https://docs.cursor.com/context/rules) for better control.

Both cursor rules and copilot's custom instructions are similar and can be written at the same time. #3562

┆Issue is synchronized with this [Notion page](https://www.notion.so/Issue-3561-DevTask-Migrate-cursorrules-to-Project-Rules-1dd6d73d36508183a194ee2329b467b3) by [Unito](https://www.unito.io)

Start by locating the existing .cursorrules file and read Cursor's linked project-rules documentation. Check the related Copilot custom-instructions issue #3562 for scope and coordination. Done means the deprecated .cursorrules setup has been migrated to the project's supported rules format.

Written by the indexing model from the issue text.