Add contribution guidelines for GitHub Copilot coding agent

Author: jaybosamiya-ms

.github/copilot-instructions.md

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+This repository contains a Rust-based, security-focused sandboxing library OS. To maintain high code quality and consistency, please adhere to the following guidelines when contributing.
+
+## Code Standards
+
+### Required Before Each Commit
+- Run `cargo fmt` to format all Rust files using `rustfmt`.
+  - This ensures consistent code style across the codebase.
+
+### Development Workflow
+The recommended sequence during development is:
+1. **Format**: `cargo fmt`
+2. **Build**: `cargo build`
+3. **Lint**: `cargo clippy --all-targets --all-features`
+4. **Test**: `cargo nextest run`
+
+- Full CI checks are defined in `.github/workflows/ci.yml`.
+
+## Key Guidelines
+
+1. Follow Rust best practices and idiomatic patterns.
+2. Preserve the existing code structure and organization.
+3. Minimize use of `unsafe` code. Every `unsafe` block **must** include a clear safety comment explaining why it's sound. Always prefer safe abstractions and code where possible.
+4. Write unit tests for new functionality, especially if it affects public interfaces.
+   - Extremely simple changes do not require explicit unit tests.
+5. Document all public APIs and non-trivial implementation details.
+6. Avoid introducing new dependencies unless strictly necessary. If a dependency is added:
+   - It must be justified.
+   - Prefer `default-features = false` in `Cargo.toml`.
+7. Favor `no_std` compatibility wherever feasible.
+   - Some crates in the workspace may use `std`, but this should be deliberate and justified.