Add comprehensive GitHub Copilot instructions for the repository

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot coding agents with all the information needed to work effectively in the Split the Trip codebase.

What's Included

The instructions provide exhaustive guidance on:

• Technology Stack: Next.js 15.4.4, React 19, TypeScript, TailwindCSS 4, Jest 30
• Validated Commands: All build, test, and development commands with actual execution times
• Manual Testing Scenarios: Complete user workflows to validate changes work correctly
• Project Structure: Key directories, files, and architectural patterns
• Build Timing Expectations: Explicit timeout values and "NEVER CANCEL" warnings

Key Features

Validated Command Set

All commands have been tested and include actual timing data:

• npm install: 30 seconds
• npm run build: 20 seconds (with 60+ minute timeout warning)
• npm test: 6 seconds (with 30+ minute timeout warning)
• npm run lint: 3 seconds

Complete Manual Testing Protocol

The instructions require agents to validate changes through specific scenarios:

1. Load example data (Pizza party $20, car rental $60, beer $15)
2. Generate payment calculations and verify results
3. Test UI functionality and responsive design

Critical Developer Information

• Static export configuration means npm start doesn't work locally
• 3 tests fail due to text mismatches (documented as expected)
• GitHub Pages deployment via Actions workflow
• Comprehensive test coverage requirements (>70%)

Screenshots

Initial Application State:

Payment Calculation Results:

The instructions follow an imperative tone ("Run this command", "NEVER CANCEL builds") and prioritize actionable information that enables coding agents to work efficiently without trial-and-error discovery.

Fixes #14.

---

💬 Share your feedback on Copilot coding agent for the chance to win a $200 gift card! Click here to start the survey.