fluentassertions — Copilot Instructions

I want to add the "fluentassertions — Copilot Instructions" prompt rules to my project.
Repository: https://github.com/fluentassertions/fluentassertions

Please read the repo to find the rules/prompt file, then:
1. Download it to the correct location (.cursorrules, .windsurfrules, .github/prompts/, or project root — based on the file type)
2. If there's an existing rules file, merge the new rules in rather than overwriting
3. Confirm what was added