Set up Copilot instructions for repository

[Copilot]

This PR adds comprehensive GitHub Copilot instructions to help the AI assistant provide better code suggestions when working on this repository.

What's Changed

Added .github/copilot-instructions.md containing repository-specific guidance for GitHub Copilot, including:

Project Context

• Overview of GoChat as a high-performance WebSocket chat server
• Key technologies (Go 1.25.1+, gorilla/websocket, GitHub Actions, Docker)
• Architecture and component descriptions (Hub, Client, Rate Limiter, etc.)

Code Standards & Best Practices

• Go style guidelines: formatting, imports organization, naming conventions, documentation requirements
• Error handling: proper error wrapping with context, avoiding panics
• Testing requirements: unit tests, table-driven tests, race detection, 80%+ coverage target
• Security practices: input validation, rate limiting, origin validation, no secrets in code

Development Workflow

• Pre-commit checklist (format, lint, test, security scan)
• Code organization and project structure
• Build and test commands reference
• Common patterns for adding handlers, configuration, and tests

Make Targets Reference

Quick reference for all development commands:

• Development: build, test, run, dev (with auto-reload)
• Code quality: fmt, lint, security-scan, vet
• CI/CD: ci-local, all
• Cross-platform: build-all, release

Documentation & Resources

• Links to all project documentation (API, Development, Contributing, Deployment, Security)
• Commit message format guidelines
• Dependency management best practices

Benefits

This configuration will help GitHub Copilot:

• Suggest code that follows the project's established patterns and conventions
• Recommend appropriate Make targets for common development tasks
• Generate tests that match the project's testing style (table-driven tests with race detection)
• Provide security-conscious suggestions (input validation, error handling, etc.)
• Understand the WebSocket architecture and suggest compatible implementations

References

• Implements best practices from GitHub Copilot documentation
• Follows the repository structure recommended by GitHub for custom instructions

Closes #11

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 #9

---

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

[Copilot]

Pull Request Overview

This PR sets up comprehensive GitHub Copilot instructions to improve AI-assisted development for the GoChat WebSocket chat server project. The instructions provide repository-specific guidance covering project context, coding standards, development workflows, and common patterns.

• Adds detailed project overview and technology stack documentation
• Establishes Go coding standards, error handling, testing, and security practices
• Documents development workflow with Make targets and CI/CD pipeline information

---

_{Tip: Customize your code reviews with copilot-instructions.md. Create the file or learn how to get started.}