docs: Add comprehensive GitHub Copilot development instructions for Gitify

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot with detailed instructions for working effectively in the Gitify codebase.

What's Included

The instructions file provides:

• Complete development workflow from initial setup to deployment
• Exact command sequences with validated timings and timeout recommendations
• Critical "NEVER CANCEL" warnings for long-running operations (builds take 30+ seconds, tests 60+ seconds)
• Validation scenarios to ensure changes work correctly
• Expected outputs and success indicators for all major commands
• Troubleshooting guidance for common environment issues

Key Features

• Imperative tone throughout ("Run this command", "Do not do this") for clear guidance
• Exhaustively validated - every single command was tested and timed in the actual environment
• Environment-aware - documents expected warnings, failures, and limitations (e.g., Electron sandbox issues in containers)
• Technology stack reference with all major dependencies and their purposes

Sample Instructions

# Bootstrap the repository
pnpm install  # takes 2.5 minutes. NEVER CANCEL. Set timeout to 5+ minutes.

# Build the application  
pnpm build    # takes 32 seconds. NEVER CANCEL. Set timeout to 60+ minutes.

# Run tests
pnpm test     # takes 67 seconds. NEVER CANCEL. Set timeout to 30+ minutes.

The instructions start with the required directive: "ALWAYS follow these instructions first and only fallback to additional search and context gathering if the information in the instructions is incomplete or found to be in error."

This will significantly improve GitHub Copilot's effectiveness when working on Gitify by providing it with the exact context it needs about build processes, timing expectations, validation steps, and common pitfalls.

Fixes #2154.

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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.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud