Add comprehensive GitHub Copilot instructions for go-configparser development

[Copilot]

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

What's Included

The instructions cover all essential aspects of development:

Quick Start & Validation

• Bootstrap commands with exact timing expectations (build: ~7s, test: ~5s)
• Essential development commands for building, testing, and code quality
• Critical "NEVER CANCEL" warnings for build operations

Comprehensive Validation Scenarios

• Working code example that tests core functionality:
  • Configuration file parsing from testdata/example.cfg
  • Basic value retrieval with Get() method
  • Interpolation testing with GetInterpolated()
• Expected output validation to ensure changes don't break functionality

Codebase Navigation

• Detailed file-by-file breakdown of the project structure
• Purpose and responsibility of each core file (configparser.go, methods.go, interpolation.go, etc.)
• Test file organization and coverage information (93.1% main package, 100% chainmap)

Development Workflows

• Step-by-step instructions for adding new features
• Debugging guidelines for configuration parsing issues
• CI/CD integration details matching the existing GitHub Actions workflow

Commands Validated

Every command in the instructions has been tested and verified:

# All of these work and have documented timing
go build -v ./...           # ~7 seconds
go test -v ./...           # ~5 seconds, 93.1% coverage  
go vet ./...               # Static analysis
gofmt -l .                 # Code formatting check
go test -check.f TestName  # Specific test execution
~/go/bin/golangci-lint run # Comprehensive linting

The validation scenario provided in the instructions successfully parses the example configuration, retrieves values, and performs interpolation, ensuring that developers can quickly verify their changes work correctly.

Benefits for GitHub Copilot

• Immediate productivity: Clear commands that work on first try
• Accurate timing expectations: Prevents premature cancellation of builds/tests
• Focused validation: Specific scenarios to test after making changes
• Codebase familiarity: Understanding of file organization and responsibilities
• Quality assurance: Linting and formatting commands to maintain code standards

Fixes #23.

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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.