Add comprehensive GitHub Copilot instructions for pycomposefile development

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot with detailed instructions for working effectively in the pycomposefile codebase.

What's Included

The instructions cover all essential aspects of development workflow:

Validated Development Commands:

• Environment setup and dependency management
• Code linting with python -m ruff check (< 1 second)
• Code formatting with python -m ruff format
• Test execution with python -m unittest discover -v -s . -p "test*.py" (85 tests, < 1 second)
• Manual functionality validation using Python code snippets

Project Structure Documentation:

• Complete overview of the /pycomposefile/ library structure
• Key files: compose_file.py, __init__.py, test organization
• Sample files location and purpose (/sample/ directory)

Network Limitation Handling:

• Documents PyPI connectivity issues in sandboxed environments
• Provides workarounds for package building failures
• Focus on local development and testing capabilities

Manual Validation Scenarios:

from pycomposefile import ComposeFile
import yaml

compose_yaml = '''
version: '3'
services:
  web:
    image: nginx:latest
    ports:
      - '80:80'
'''
parsed = yaml.load(compose_yaml, Loader=yaml.Loader)
compose_file = ComposeFile(parsed)

CI Requirements:

• Exact commands that must pass before committing
• Matches .github/workflows/build.yaml requirements
• Clear validation steps for code quality

Validation

All commands and code snippets in the instructions have been thoroughly tested and validated to work correctly in the development environment. The instructions prioritize commands that work reliably while documenting known limitations with appropriate workarounds.

Fixes #46.

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