✨ Set up Copilot instructions and complete project packaging infrastructure

[Copilot]

This PR implements a comprehensive setup for Copilot instructions and modern Python packaging infrastructure as requested in the issue. The changes transform the simple single-file project into a fully-featured, pip-installable package with proper CI/CD integration.

Key Changes

Copilot Instructions

• Added .github/copilot-instructions.md following best practices for GitHub Copilot coding agent
• Provides clear guidelines for development, testing, code style, and project structure
• Documents all supported features and usage patterns

Python Package Structure

• Restructured project from single main.py to proper package layout under src/interactive_sort/
• Created pyproject.toml for modern Python packaging with setuptools
• Added entry point for command-line usage: interactive-sort
• Package is now pip-installable and can be run as both interactive-sort and python -m interactive_sort

Testing Infrastructure

• Set up comprehensive test suite with pytest covering all sorting modes
• Added tox.ini for multi-Python version testing (3.8-3.12)
• Configured test environments for linting, formatting, and coverage reporting

Code Quality & Formatting

• Configured ruff for code formatting with 120 character line length as requested
• Fixed all linting issues and applied consistent formatting
• Added type hints and improved code documentation

CI/CD Pipeline

• Created .github/workflows/ci.yml that runs tox on every push to master branch
• Tests across multiple Python versions (3.8, 3.9, 3.10, 3.11, 3.12)
• Includes separate jobs for linting and formatting validation
• Integrated with tox-gh-actions for efficient GitHub Actions execution

Documentation & Project Files

• Created comprehensive README.md with installation and usage instructions
• Added examples for all sorting features (numeric, human-readable, month-based, etc.)
• Included development setup and contribution guidelines
• Added MIT LICENSE file and proper .gitignore

Installation & Usage

After these changes, users can install and use the tool as follows:

# Install from source
pip install .

# Use as command-line tool
interactive-sort --help
echo -e "banana\napple\ncherry" | interactive-sort

# Use as Python module
python -m interactive_sort --numeric-sort

The tool maintains all existing functionality while now being properly packaged for distribution. The interactive sorting works exactly as before, with real-time display as users type lines of text.

Testing

All tests pass with good coverage:

• 10 unit tests covering all sorting modes and edge cases
• Tests for alphabetical, case-insensitive, numeric, human-readable, and month-based sorting
• Verification of field separator functionality and key generation

The project now follows modern Python packaging standards and is ready for PyPI distribution if desired.

Fixes #3.

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