Add comprehensive GitHub Copilot instructions for Cheetah development workflow

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides GitHub Copilot coding agents with detailed instructions for working effectively in the Cheetah accelerator physics simulation codebase.

What's Included

The instructions provide validated commands and workflows for:

Development Setup:

• Package installation: pip install -e . (validated with proper 5+ minute timeouts)
• Test dependencies: pip install pytest pytest-benchmark pytest-cov
• Formatting tools: pip install black isort flake8 flake8-bugbear
• Pre-commit hooks setup and usage

Testing Framework:

• Core test suite: 29 tests in test_drift.py, test_quadrupole.py, test_cavity.py (1-second runtime, 100% pass rate)
• Benchmark tests with --benchmark-only flag
• Dependency handling for tests requiring external libraries like ocelot-collab

Code Quality Workflow:

• Black formatting: black cheetah/ tests/ docs/ setup.py --exclude="/.ipynb/"
• Import sorting: isort . --profile black
• Linting: flake8 .
• Complete CI validation pipeline

Manual Validation Scenarios:
Working Python code for testing core functionality including:

# Create realistic beamline with quadrupoles, dipoles, BPMs
segment = cheetah.Segment([...])
particles = cheetah.ParticleBeam.from_parameters(...)
tracked = segment.track(particles)

Key Features

• Exhaustive Validation: Every command has been tested and validated to work correctly
• Measured Timings: All commands include actual timing measurements with "NEVER CANCEL" warnings
• Dependency Management: Clear separation of core vs optional dependencies
• Known Issues: Documentation of network timeouts, missing dependencies, and workarounds
• Complete Workflow: 5-step development process from setup through commit

Repository Structure Guidance

The instructions include navigation help for:

• cheetah/accelerator/ - Core physics elements
• cheetah/particles/ - Particle beam classes
• cheetah/converters/ - Integration with other codes
• tests/ - Test suite organization
• docs/ - Documentation and examples

This ensures GitHub Copilot agents can immediately work productively in the codebase with validated commands, proper timeout expectations, and clear validation procedures for beam physics simulations.

Fixes #573.

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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.