Add copilot-instructions

[sisuresh]

Description

Adds copilot instructions to comment if fee bumps tests should be written.

We could also make additional changes for more targeted reviews on things like style guidelines and missing tests.

Here's an example of this in action - sisuresh#15 (comment)

Checklist

• Reviewed the contributing document
• Rebased on top of master (no merge commits)
• Ran clang-format v8.0.0 (via make format or the Visual Studio extension)
• Compiles
• Ran all tests
• If change impacts performance, include supporting evidence per the performance document

[Copilot]

Pull Request Overview

This PR adds GitHub Copilot instructions to the repository to help improve code review quality by providing specific guidance on test coverage requirements. The instructions focus on ensuring that fee bump tests are written alongside regular transaction tests.

• Adds .github/copilot-instructions.md file with fee bump testing guidelines
• Provides automated guidance for code reviewers about test coverage expectations

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_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}