Add comprehensive GitHub Copilot instructions for DeviceKit development workflow

[Copilot]

This PR adds a comprehensive .github/copilot-instructions.md file that provides detailed guidance for GitHub Copilot and developers working with the DeviceKit codebase.

Key Features Added

Platform Requirements & Limitations

• Clear documentation that DeviceKit requires macOS with Xcode (cannot build on Linux)
• Specific warnings about ProcessInfo API incompatibilities on non-Apple platforms

Critical Code Generation Rules

• Explicit warnings about never editing Device.generated.swift directly
• Mandatory use of the official gyb command: ./Utils/gyb --line-directive '' -o ./Source/Device.generated.swift ./Source/Device.swift.gyb
• Required pre-commit code generation step to ensure generated files are up-to-date

Complete Development Workflow

• Step-by-step bootstrap instructions for fresh repository clones
• Code generation process using the official gyb executable to create Device.generated.swift
• Multiple build system support (Swift Package Manager, Xcode, CocoaPods)
• Comprehensive test execution guidelines

Validation & Testing Scenarios

• Device detection library-specific validation approaches
• Multi-platform simulator testing instructions
• Playground setup and usage guidance
• Enhanced pre-commit validation steps (code generation, SwiftLint, tests)

Time Expectations & Performance

• Detailed timeout recommendations with "NEVER CANCEL" warnings
• Realistic time estimates for builds (2-5 minutes) and tests (1-3 minutes)
• Specific guidance for CI pipeline expectations

Troubleshooting & Common Issues

• Platform-specific error solutions
• Ruby gem installation troubleshooting
• Xcode scheme and simulator setup guidance
• Code generation validation steps with correct command syntax

Developer Workflow Patterns

• Adding new device support process with proper code generation workflow
• Testing changes across multiple platforms
• Essential pre-commit validation checklist including mandatory code generation

The instructions follow an imperative tone as requested and include validation status indicating which commands were verified on Linux vs. those requiring macOS/Xcode. This ensures developers and Copilot have accurate, actionable guidance for working effectively with the DeviceKit codebase.

Fixes #453.

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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.

[Copilot]

@Zandor300 👋 This repository doesn't have Copilot instructions. With Copilot instructions, I can understand the repository better, work faster and produce higher quality PRs.

I can generate a .github/copilot-instructions.md file for you automatically. Click here to open a pre-filled issue and assign it to me. I'll write the instructions, and then tag you for review.

[github-actions]

	1 Error
🚫	This PR has no changes at all, this is likely an issue during development.

	2 Warnings
⚠️	PR is classed as Work in Progress
⚠️	Plist changed, don't forget to localize your plist values

Generated by 🚫 Danger

[Zandor300]

@copilot

Some instructions for you to add to the repository for you to use in the future:

File Source/Device.generated.swift may never be edited directly. We always apply changes to Source/Device.swift.gyb and then run the following command to generate the Device.generated.swift file:

./Utils/gyb --line-directive '' -o ./Source/Device.generated.swift ./Source/Device.swift.gyb

So everytime you commit, you should run that right before to update the Device.generated.swift file with your gyb changes.

[Copilot]

@copilot

Some instructions for you to add to the repository for you to use in the future:
...

Updated the copilot instructions to use the correct gyb command and added critical rules about never editing Device.generated.swift directly. The instructions now emphasize using ./Utils/gyb --line-directive '' -o ./Source/Device.generated.swift ./Source/Device.swift.gyb and running this command before every commit. All command references throughout the file have been updated accordingly. (7c98b15)