Use Copilot instructions

[xperiandri]

Proposed Changes

Added repository-level Copilot guidance in .github/copilot-instructions.md, including project structure, build and test commands, package/version management rules, and F# coding and testing conventions. Added a reusable agent prompt at .github/prompts/create-pr-description.prompt.md that defines a strict workflow for generating pull request descriptions from origin/main...HEAD diff and commit evidence. The prompt enforces use of the project PR template structure, checkbox selection rules, and evidence-based checklist evaluation. It also defines output formatting constraints so generated descriptions are ready to paste into GitHub without extra edits.

Types of changes

What types of changes does your code introduce to FSharp.Control.R3?
Put an x in the boxes that apply

• Bugfix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)

Checklist

Put an x in the boxes that apply. You can also fill these out after creating the PR. If you're unsure about any of them, don't hesitate to ask. We're here to help! This is simply a reminder of what we are going to look for before merging your code.

• Build and tests pass locally (not visible from diff or commit history)
• I have added tests that prove my fix is effective or that my feature works (if appropriate) (no test file changes in this branch)
• I have added necessary documentation (if appropriate)