✨ Set up Copilot instructions for Zephyr repository

[Copilot]

This PR implements comprehensive Copilot instructions for the Zephyr repository as requested in issue #XX. The instructions are designed to help AI-assisted development maintain consistency with Zephyr's established patterns and architecture.

What's Added

A new .github/copilot-instructions.md file that provides:

Project-Specific Guidance

• Architecture Overview: Detailed explanation of Zephyr's modular design with modules, profiles, and the zephyrctl controller
• Directory Structure: Clear mapping of how components are organized across bin/, profiles/, and category-based module directories
• Core Components: Documentation of key files like zephyrctl, library.sh, bootstrap.sh, and the module ecosystem

Development Standards

• Bash Scripting Guidelines: Strict mode usage (set -eufo pipefail), consistent function naming (kebab-case), and proper error handling
• Module Development Patterns: Standardized templates and best practices for creating new customization modules
• Configuration Management: How to properly handle INI-style configs, global/local variants, and profile inheritance

Quality Assurance

• Testing Requirements: Integration with existing CI pipeline, idempotent module design, and the dual-pass testing approach
• Security Practices: Safe privilege management, input validation, and secure temporary file handling
• Output Consistency: Proper use of print-status, print-error, and other library functions for uniform user experience

Practical Examples

The instructions include working code templates based on actual repository patterns:

# Proper module structure with error handling
source "${PROJECT_ROOT}/bin/bootstrap.sh"
check-not-root

if ! is-empty-section "${config_file}" "section_name"; then
    for line in $(cfg-read "${config_file}" "section_name"); do
        # Process configuration...
    done
fi

Why This Matters

Zephyr is a sophisticated Ubuntu customization framework with specific architectural patterns and conventions. These instructions ensure that AI-assisted development:

1. Maintains Consistency: Follows established naming, structure, and coding patterns
2. Preserves Security: Respects privilege boundaries and safe file handling practices
3. Supports Maintainability: Uses the existing library functions and testing infrastructure
4. Enhances User Experience: Maintains consistent output formatting and error handling

The instructions are tailored specifically to Zephyr's Bash-based architecture and modular design, providing context that generic coding guidelines cannot offer.

Original prompt

---

This section details on the original issue you should resolve

<issue_title>✨ Set up Copilot instructions</issue_title>
<issue_description>Configure instructions for this repository as documented in Best practices for Copilot coding agent in your repository.

</issue_description>

Comments on the Issue (you are @copilot in this section)

Fixes #215

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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.