Add comprehensive GitHub Copilot instructions for go-workflows development

[Copilot]

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

What's Included

The instructions cover all essential development workflows with validated commands and measured timing expectations:

Build and Test Setup

• Complete dependency installation (go mod download - 3.5 minutes)
• Full build process (go build -v ./... - 40 seconds)
• Test execution with proper timeouts (short tests - 45 seconds, full suite - 2.5 minutes)
• Docker Compose setup for development dependencies (13 seconds)

Development Tools

• Analyzer plugin compilation for custom linting rules
• golangci-lint setup and execution
• JUnit test reporting configuration

Validation Scenarios

Specific manual testing scenarios to verify workflow functionality:

• Basic end-to-end workflow execution
• Multi-backend testing (Redis, SQLite, MySQL)
• Complex workflow patterns (subworkflows, timers, signals)
• Sample application verification

Repository Navigation

• Key directory structure and purposes
• Important files and configuration
• Backend implementations and their use cases
• Sample applications demonstrating different patterns

Critical Features

Timeout Specifications: Every long-running command includes explicit timeout recommendations and "NEVER CANCEL" warnings to prevent premature termination of builds that can take several minutes.

Validated Commands: All instructions have been tested and validated to work correctly in the current codebase state, including handling of known issues like analyzer plugin version conflicts.

Practical Examples: Real workflow execution examples with expected outputs to help agents verify their changes work correctly.

This enables GitHub Copilot agents to work effectively in the codebase without extensive exploration, reducing development time and preventing common setup mistakes.

Fixes #400.

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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.