add copilot instructions at the repo level

[levalleux-ludo]

the instruction should specify the naming rule for pull request and commits, as checked by the GH action lint-pr.yaml

[levalleux-ludo]

Setting Rules for Pull Request Naming in GitHub Copilot
Custom Instructions
To guide GitHub Copilot on pull request (PR) naming conventions, you can use custom instructions. Here’s how to set them up:

Create a copilot-instructions.md file: This file should contain general guidelines for your repository.
Use path-specific instructions: For more detailed rules, create *.instructions.md files in the .github/instructions directory. Use the applyTo frontmatter to target specific languages or areas.
Example Rules to Include
Format: Specify the required format for PR titles, such as "feature/description" or "fix/issue".
Length: Set minimum and maximum character limits for titles.
Prefixes: Define allowed prefixes (e.g., "feature", "fix") and disallowed prefixes (e.g., "feat/").