Add comprehensive GitHub Copilot instructions for git-artifact repository

[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 the git-artifact codebase.

Key Features

Complete Development Workflow Coverage:

• Environment setup and installation (Bash 4.3+, Git, PATH configuration)
• Testing procedures with precise timing expectations
• Linting with shellcheck
• Manual validation scenarios
• Docker-based multi-version testing

Validated Commands and Timings:
All commands have been thoroughly tested and include realistic timing expectations:

• Full test suite: ~60 seconds (NEVER CANCEL)
• Individual test cases: 7-15 seconds each
• Shellcheck linting: ~2 seconds
• Core git-artifact operations: <1 second each

Comprehensive Validation Requirements:
The instructions mandate complete validation after any changes:

1. Shellcheck linting (2 seconds)
2. Single test validation (7 seconds)
3. Manual functional testing (30 seconds)
4. Full test suite (60 seconds)

Key Sections Include:

• Overview of git-artifact tool and its "horizontal" git history approach
• Step-by-step installation and setup
• Complete testing procedures with explicit timeout values
• Manual functional validation with working code examples
• Repository structure explanation
• CI/CD integration notes
• Troubleshooting guidance

The instructions emphasize the "NEVER CANCEL" principle for long-running operations and provide agents with everything needed to work confidently in this Bash-based CLI tool repository.

Fixes #36.

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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.