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

Key Features

Critical Build Time Guidance:

• Explicit "NEVER CANCEL" warnings for all major build operations
• Specific timeout recommendations (60+ to 300+ minutes) aligned with CI configuration
• Build time estimates for all hardware targets based on documented workflows

Complete Hardware Coverage:

• x86_64 targets (VM, generic, Lenovo X1, NVIDIA Jetson cross-builds)
• ARM64 targets (NVIDIA Jetson AGX/NX, i.MX 8MP-EVK)
• RISC-V targets (Microchip Icicle Kit)
• Installer image builds with specific requirements

Validation Scenarios:

• VM testing as primary validation method with specific cleanup procedures
• Hardware-specific testing scenarios (recovery mode flashing, serial console setup, etc.)
• Post-build validation checklist with format checks and license validation

Development Workflow:

• Prerequisites including Nix installation and flakes configuration
• Code quality checks (nix fmt, reuse lint) that must pass for CI
• Security and compliance requirements with license header templates
• Troubleshooting guidance for common build and runtime issues

Repository Navigation:

• Key file locations and directory structure explanation
• Development tools and testing modules documentation
• Cross-compilation setup requirements with remote builder configuration

The instructions follow an imperative tone throughout ("Run this command", "Always do this") and include a firm directive to reference these instructions first before searching or exploring the codebase independently.

All commands and paths have been validated against the existing repository structure and documentation to ensure accuracy and completeness. The file has been properly added to REUSE.toml for license compliance.

Fixes #1356.

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