chore: add copilot instructions

.github/copilot-instructions.md

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+# Copilot Coding Agent Instructions for Koenkk/zigbee2mqtt
+
+Welcome! These instructions help ensure Copilot Coding Agent can efficiently collaborate on this repository.
+
+## 1. Codebase Overview
+- **Main language:** JavaScript (Node.js), TypeScript
+- **Purpose:** Zigbee to MQTT bridge for home automation.
+- **Core directories:**
+  - `src/`: Main source code (device adapters, communication, logic).
+  - `test/`: Automated tests.
+  - `docs/`: Documentation.
+  - `data/`: Zigbee device definitions.
+
+## 2. Preferred Practices
+- **Branching:** Use feature branches (`feat/xyz`) for enhancements and `fix/xyz` for bug fixes.
+- **Commits:** Write clear, descriptive commit messages (imperative mood, < 72 chars).
+- **Pull Requests:** Reference related issues, provide context, and include before/after behavior if modifying logic. Tag with appropriate labels.
+- **Testing:** All code changes should include or update relevant tests. A test coverage of 100% is enforced. Run `npm test:coverage` before submitting PRs.
+- **Linting:** Code must pass lint and formatting checks (`npm run check`). Use Biome for formatting.
+- **Documentation:** Update relevant docs when adding features or changing behavior.
+
+## 3. Review & Feedback
+- All PRs require review by maintainers.
+- Automated checks must pass before merging.
+- If you’re fixing a bug, include steps to reproduce in the PR description.
+
+## 4. Security & Secrets
+- Do not commit secrets, credentials, or private keys.
+- Follow the repository’s security policy for vulnerability disclosures.
+
+## 5. Communication
+- Use Discussions and Issues for questions and proposals.
+- Respect community guidelines and code of conduct.
+
+## 6. Special Instructions for Copilot Coding Agent
+- Suggest code changes that strictly adhere to existing styles and patterns.
+- Explain reasoning in PR descriptions when implementing complex changes.
+- Prioritize backward compatibility unless otherwise specified.
+- If uncertain, prompt for clarification via PR comment before proceeding.
+
+---
+
+For more details, see [Best practices for Copilot coding agent in your repository](https://gh.io/copilot-coding-agent-tips) and the repository's CONTRIBUTING.md.