Add comprehensive GitHub Copilot instructions for GitLang repository

[Copilot]

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

What's Included

The instructions cover all essential aspects for developers working with a fresh clone:

Build & Development Workflow

• Exact commands for dependency installation (npm install - 35-45s)
• Development build (npm run build:dev - 3-5s) vs production build (npm run build:prod - 8-12s)
• Dual server setup: API server on port 3000, client server on port 8080
• Critical timeout specifications with "NEVER CANCEL" warnings for all long-running operations

Environment Setup

• GitHub PAT requirements with public_repo scope
• .env file configuration for API authentication
• Security vulnerability management via npm audit fix

Manual Validation Scenarios

• Step-by-step UI testing procedures at http://localhost:8080
• Search functionality validation (username input → language charts)
• Expected behavior with/without valid GitHub tokens
• Screenshot verification requirements for UI changes

Repository Architecture

• TypeScript/Svelte frontend with Node.js/Koa backend structure
• Key directories: /client/, /server/, /local/, /public/
• API endpoints and authentication flow
• AWS deployment pipeline integration

Code Quality & Troubleshooting

• ESLint configuration with auto-fix (npm run fix - 10-15s)
• Known issues: 21 linting warnings (non-blocking), 1 remaining security vulnerability
• Common CORS and authentication error patterns

Validation Completed

All instructions were thoroughly tested and validated:

• ✅ Complete dependency installation and build process
• ✅ Both development and production builds successfully tested
• ✅ Local servers start and function correctly
• ✅ Web application UI manually validated with screenshots
• ✅ Security fixes applied (reduced from 13 to 1 vulnerability)
• ✅ All timing measurements verified with appropriate timeout recommendations

The instructions follow an imperative tone as requested and provide explicit timing expectations to prevent premature cancellation of builds or long-running commands.

Fixes #895.

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