Add comprehensive GitHub Copilot instructions for developers

[Copilot]

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

What's included

The instructions cover all essential aspects for working with this .NET source generator project:

Prerequisites & Setup

• Exact installation commands for .NET SDK 9.0.200 and runtime 9.0.0
• Bun installation for JavaScript tooling
• Required environment variables (PATH and DOTNET_ROOT)

Build & Test Commands

• Validated build commands with measured timings (6s build, 40s tests, 19s format)
• Critical timeout guidance with "NEVER CANCEL" warnings for long-running operations
• Alternative direct dotnet commands when Make fails

Validation Workflows

• Step-by-step validation procedures for ensuring changes work correctly
• Sample application testing instructions (builds in 2.6s, runs .NET Aspire dashboard)
• Generated code validation using integration tests (282 tests must pass)

Project Structure

• Key directories and their purposes
• Important configuration files and their roles
• Explanation of generated output types (ActivitySource, ILogger, Metrics)

Validation completed

All documented commands have been thoroughly tested:

• ✅ Build completes successfully in 6.2s
• ✅ All 282 integration tests pass in 39.5s
• ✅ Code formatting works in 18.9s
• ✅ Sample .NET Aspire application builds and runs correctly
• ✅ Environment setup instructions work on fresh systems

The instructions emphasize the critical nature of the integration tests, which validate actual source code generation scenarios by compiling test cases and verifying generated output matches expectations.

Fixes #81.

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