Add GitHub Copilot instructions for repository

[Copilot]

Configures repository-specific context for GitHub Copilot as requested in the issue.

Changes

• Added .github/copilot-instructions.md with comprehensive guidance covering:
  • OSGi bundle architecture and component patterns
  • Maven build commands and Spotless formatting requirements
  • Java 21 + JUnit 5 testing conventions
  • Contribution workflow emphasizing git rebase over git merge
  • Common code patterns for Declarative Services and configuration handling
  • Project structure navigation for 100+ core bundles

The file provides Copilot with necessary context about the openHAB Core framework to generate appropriate suggestions for OSGi development, Maven builds, and project-specific conventions.

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 #5145

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.