✨ Enhance Copilot instructions with comprehensive best practices

[Copilot]

• Review current copilot-instructions.md and identify gaps based on best practices
• Add project overview and technology stack section
• Add project structure section
• Add build commands and dependencies section
• Add testing commands section
• Add linting and formatting commands section
• Reorganize content to follow GitHub Copilot best practices
• Maintain all original domain-specific security guidance
• Verify no build or test issues - all formatting checks pass
• Address PR feedback:
  • Clarify cross-compilation limitations and correct documentation reference
  • Add cargo doc to required checks before commit
  • Clarify that default test commands won't run anything and mention nextest config
  • Better clarify trust boundaries between OpenVMM, OpenHCL, root, and VTL0 guest

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. There may already be instructions that were previously set up, but the best practices have likely changed since then.

</issue_description>

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

Fixes #2220

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💡 You can make Copilot smarter by setting up custom instructions, customizing its development environment and configuring Model Context Protocol (MCP) servers. Learn more Copilot coding agent tips in the docs.

[Copilot]

Pull Request Overview

This PR enhances the GitHub Copilot instructions file to provide comprehensive context and guidance for the OpenVMM repository, expanding it from 29 to 104 lines following GitHub's best practices for Copilot coding agents.

Key Changes:

• Added structured sections for project overview, technology stack, and project structure to provide better repository context
• Expanded build, test, and formatting sections with concrete commands and examples
• Reorganized existing domain-specific security guidelines into a clearer structure

[mattkur]

1. You can't build certain components from Windows (even if you're targeting Linux), you must use Linux. WSL counts as linux.
2. restore-packages --help doesn't help with cross-compile. Look at the Guide (and see references for . ./build_support/setup_windows_cross.sh as an example)

[Copilot]

Updated cross-compilation section to clarify that some components like OpenHCL can only be built from Linux (WSL2 counts as Linux), and corrected the reference to point to the Guide and the setup_windows_cross.sh script. (commit c5b3738)

[mattkur]

You also want to run cargo doc before committing, to catch any errors in the rustdocs.

[Copilot]

Added cargo doc to the required checks before committing to catch rustdoc errors. (commit c5b3738)

[mattkur]

Note that the default won't actually run anything. You should run the tests in the packages that you are modifying. vmm-tests is a special case and requires additional setup.

[mattkur]

You likely also want to use the /.config/nextest.toml file to configure how you run tests.

[Copilot]

Updated testing section to clarify that default won't run anything and users should specify packages with -p <package-name>. Also added a note about using .config/nextest.toml for test configuration. (commit c5b3738)

[mattkur]

This has tripped me up with copilot-generated and copilot-reviewed code. We should better clarify the trust boundary:

• OpenVMM does not trust the VTL0 guest.
• OpenHCL does not trust the root.
• OpenHCL does not trust the VTL0 guest, but this is less critical than OpenVMM not trusting the VTL0 guest. (the attack surface is subtle, so needs human review).

[Copilot]

Added explicit trust boundary clarifications:

• OpenVMM does not trust the VTL0 guest
• OpenHCL does not trust the root
• OpenHCL does not trust the VTL0 guest (less critical, subtle attack surface needs human review)

This should help Copilot better understand and respect these critical security boundaries. (commit c5b3738)

[mattkur]

@copilot, I left some comments for you.