chore: add GitHub Copilot instructions

[Copilot]

Added .github/copilot-instructions.md to provide GitHub Copilot with repository-wide context following best practices from https://gh.io/copilot-coding-agent-tips.

Content

The instruction file consolidates:

• Project structure - Component mapping (backend, website, integration-tests, preprocessing, ingest, kubernetes, etc.)
• Commit conventions - Conventional commits format with component scopes and breaking change markers
• Testing/building - Component-specific commands for Kotlin backend, TypeScript website, and Playwright tests
• Code style - Clean code principles and project-specific patterns
• Website-specific - Playwright test flake prevention (hydration-aware components)
• References - Links to component-specific AGENTS.md files and key documentation

Screenshot

N/A - Documentation only change

PR Checklist

• All necessary documentation has been adapted.
• The implemented feature is covered by appropriate, automated tests.
• Any manual testing that has been done is documented (i.e. what exactly was tested?)

Manual verification: Confirmed .github/copilot-instructions.md created with 167 lines covering all major repository aspects.

Original prompt

---

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 #5786

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