Add comprehensive GitHub Copilot instructions for Meshroom development

[Copilot]

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

What's Included

The instructions cover all essential development workflows with fully validated commands:

Setup and Dependencies

• Python environment setup (3.9+) with exact dependency installation steps
• Critical PYTHONPATH=$PWD configuration requirement
• Development tools installation (flake8, pytest-cov)

Build and Test Process

• Exact CI linting commands that match the GitHub Actions workflow
• Unit testing with pytest (116 tests, ~0.79s execution time)
• Documentation building with Sphinx (~31s)
• Package building with cx_Freeze (~2 minutes)

Validation and Quality Assurance

• Pre-commit validation steps (linting + full test suite)
• Manual testing scenarios for CLI functionality and core imports
• Expected results (116 tests pass, 236 style warnings are normal)

Timing and Timeout Guidelines

• Every command was measured and validated during development
• Appropriate timeout recommendations (10+ minutes for safety buffers)
• "NEVER CANCEL" warnings for potentially long-running operations
• Actual timing data: dependencies (~30s), tests (~1s), docs (~31s)

Environment Configuration

• Essential environment variables (PYTHONPATH, ALICEVISION_ROOT)
• Optional AliceVision integration for full 3D reconstruction functionality
• Platform-specific notes (Linux/Windows/macOS)

Validation Process

All instructions were exhaustively validated by:

1. Creating a fresh repository clone
2. Following every command step-by-step
3. Verifying expected outputs and timing
4. Testing core functionality (CLI tools, imports, test suite)

The instructions follow the imperative tone requirement ("Run this command", "Set timeout to X minutes") and provide reliable, copy-pasteable commands that work consistently.

Repository Structure Reference

The instructions include a complete overview of the codebase:

• Core Python package (meshroom/)
• CLI tools (bin/ directory)
• Testing framework (tests/ with pytest)
• Documentation system (docs/ with Sphinx)
• CI/CD configuration (.github/workflows/)

This enables GitHub Copilot to understand the project structure and provide context-aware assistance for development tasks.

Fixes #2830.

Warning

Firewall rules blocked me from connecting to one or more addresses

I tried to connect to the following addresses, but was blocked by firewall rules:

• esm.ubuntu.com
  • Triggering command: /usr/lib/apt/methods/https (dns block)

If you need me to access, download, or install something from one of these locations, you can either:

• Configure Actions setup steps to set up my environment, which run before the firewall is enabled
• Add the appropriate URLs or hosts to the custom allowlist in this repository's Copilot coding agent settings (admins only)

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✨ Let Copilot coding agent set things up for you — coding agent works faster and does higher quality work when set up for your repo.