Add Copilot instructions for repository

[Copilot]

Configures GitHub Copilot Coding Agent instructions per best practices to improve autonomous code generation quality.

Changes

Added .github/copilot-instructions.md (226 lines) documenting:

• Project context: Docker-based Windows virtualization via QEMU/KVM, supports Windows 2000-11 and Server editions
• Tech stack: Bash scripts (4600+ lines), Docker multi-stage builds, XML unattended configs
• Coding standards:
  • Shell: set -Eeuo pipefail, ShellCheck with 8 exclusions (SC1091, SC2001, SC2002, SC2034, SC2064, SC2153, SC2317, SC2028)
  • Docker: hadolint compliance, ignoring DL3006/DL3008
  • Config: XML/YAML validation in CI
• Build process: ShellCheck, hadolint, XML validation commands matching CI workflows
• Domain knowledge: Windows version mapping (define.sh), automatic installation flow, environment variables, QEMU/virtualization concepts
• Critical constraints: Stability over 200+ Windows version/edition combinations, minimal changes to interdependent scripts

Enables agent to understand specialized virtualization domain, shell script interdependencies, and stability requirements when making changes.

Original prompt

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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 ✨ Set up Copilot instructions #3

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