Add comprehensive GitHub Copilot instructions for PostgreSQL Query Builder

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot coding agents with detailed guidance on how to work effectively in the PostgreSQL Query Builder codebase.

What's Included

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

• Setup & Dependencies: Exact commands for npm install with timing expectations (~2-3 seconds)
• No Build Required: Clear guidance that this is pure JavaScript with no build step
• Testing Strategy: Since no formal test suite exists, detailed manual validation approaches including:
  • Running the provided examples (queryBuilder.examples.js)
  • Creating custom validation scripts in /tmp
  • Testing both PostgreSQL and MySQL dialects via DB_DIALECT environment variable
• Repository Structure: Overview of key files and their purposes
• Development Guidelines: Best practices for making changes with emphasis on manual validation
• Comprehensive Validation: End-to-end test script for thorough functionality verification

Key Features

• Imperative tone throughout ("Run this command", "Do not do this")
• All commands validated - every single command in the instructions has been tested to ensure it works
• Database dialect support - Clear instructions for testing both PostgreSQL ($1, $2, ...) and MySQL (?) parameter binding
• Manual testing patterns - Practical examples since no formal test infrastructure exists
• Quick reference commands - Copy-pasteable validation commands for common scenarios

Example Usage

The instructions enable agents to quickly validate changes:

# Basic functionality test
node -e "const QB=require('./queryBuilder'); const q=new QB.QueryBuilder().select('*').from('users').sql(); console.log('✓ Works:', !!q.sql)"

# MySQL dialect test  
DB_DIALECT=mysql node -e "const QB=require('./queryBuilder'); const q=new QB.QueryBuilder().select('*').whereEquals('id',1).sql(); console.log('MySQL:', q)"

The comprehensive validation script tests library loading, instance creation, SQL generation, parameter binding, and JOIN functionality - providing confidence that changes don't break core functionality.

Fixes #5.

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