[WIP] Configure Copilot instructions for the repository

[Copilot]

Plan: Set up Copilot Instructions

• Create .github directory structure
• Create .github/copilot-instructions.md file with:
  • Project overview and purpose
  • Technology stack (Lua, Reaper DAW plugin)
  • Project structure and module organization
  • Coding conventions and style guidelines
  • Testing approach (if any)
  • Build/installation instructions
  • Documentation standards
  • Future roadmap context
• Review and validate the instructions file

Original prompt

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This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

</issue_description>

Comments on the Issue (you are @copilot in this section)

• Fixes ✨ Set up Copilot instructions #13

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.