This PR adds comprehensive GitHub Copilot instructions to help provide better context-aware suggestions when working on this repository.

Changes

Added .github/copilot-instructions.md with detailed guidance including:

• Project Overview: Description of the Slack CLI tool and its purpose as a simple, agent-friendly command-line interface for sending Slack messages
• Development Setup: Prerequisites (Go 1.24.4+), building instructions, and testing guidelines
• Code Style & Conventions: Go best practices and project-specific guidelines
• Key Features: Current functionality (send-message command) and design principles (simplicity, zero runtime dependencies, graceful shutdown)
• Common Patterns: Guidelines for adding new commands and error handling patterns
• Release Process: Documentation of the automated release workflow
• Copilot-Specific Notes: Guidance on maintaining backward compatibility and reliability

Benefits

The Copilot instructions file will help GitHub Copilot:

• Understand the project's architecture and design philosophy
• Provide suggestions consistent with the codebase's style and conventions
• Maintain the minimal, agent-friendly design approach
• Suggest appropriate error handling and testing patterns
• Consider the production use case by coding agents

This follows the best practices outlined in GitHub's Copilot coding agent documentation.

Fixes #3

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