ci: add .github/copilot-instructions.md (#1658)

This change adds a copilot-instructions.md file which will be used by
@copilot to give more context on how to develop for the openvmm repo.

See:
https://docs.github.com/en/enterprise-cloud@latest/copilot/how-tos/agents/copilot-coding-agent/best-practices-for-using-copilot-to-work-on-tasks

---------

Co-authored-by: Ben Hillis <benhill@ntdev.microsoft.com>
Co-authored-by: Matt LaFayette (Kurjanowicz) <mattkur@microsoft.com>

Author: 3 people

.github/copilot-instructions.md

@@ -0,0 +1,29 @@
+This is a Rust based repository. Please follow these guidelines when contributing:
+
+## Code Standards
+
+### Required Before Each Commit
+- Run `cargo xtask fmt --fix` before committing any changes to ensure proper code formatting.
+- This will ensure all source code and generated pipeline files maintain consistent style and content.
+- Cross-compile by targeting `x86_64` and `aarch64` processors, and Windows and Linux environments.
+
+## Key Guidelines
+1. Follow Rust best practices and idiomatic patterns.
+2. Maintain existing code structure and organization.
+3. Write unit tests for new functionality.
+4. Document public APIs and complex logic. Suggest changes to the `Guide/` folder when appropriate.
+
+## Domain-specific Guidelines
+Both OpenVMM and OpenHCL processes data from untrusted sources. OpenHCL runs in a constrained environment.
+
+When possible:
+1. Avoid `unsafe` code.
+2. Avoid taking new external dependencies, or those that can significantly increase binary size.
+
+## Testing
+The OpenVMM project contains several types of tests.
+- **Unit tests** are spread throughout crates, and are marked by a `#[cfg(test)]` code block.
+- **VMM tests** are integration tests. These are found in the `vmm_tests` folder, and use the code in `petri` as a framework to help create Hyper-V and OpenVMM based VMs.
+- **Fuzz tests** are nondeterminstic, and are used to ensure that the code does not panic across trust boundaries.
+
+Whenever possible, thoroughly test the code with unit tests. Add a test case to the VMM tests if there is an interesting integration point.