Improve setup for GitHub Copilot coding agent (#170)

By default, the coding agent doesn't seem to know that it should set up
(say) the TUN device, or install nextest, and it needs to spend time
figuring it out, or failing to do so. The
`.github/workflows/copilot-setup-steps.yml` sets these things up in
advance.

Additionally, it noticeably seems to forget to format code, so I've
added `.github/copilot-instructions.md` that tells it to do this for
each commit. I also gave it some more information, which should
hopefully send it down a nicer path that better matches workflow within
our repo.

Author: jaybosamiya-ms

.github/copilot-instructions.md

@@ -0,0 +1,30 @@
+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.

.github/workflows/copilot-setup-steps.yml

@@ -0,0 +1,39 @@
+# Customize GitHub Copilot coding agent development environment
+name: "Copilot Setup Steps"
+
+# Automatically run the setup steps when they are changed to allow for easy validation, and
+# allow manual testing through the repository's "Actions" tab
+on:
+  workflow_dispatch:
+  push:
+    paths:
+      - .github/workflows/copilot-setup-steps.yml
+  pull_request:
+    paths:
+      - .github/workflows/copilot-setup-steps.yml
+
+jobs:
+  # The job MUST be called `copilot-setup-steps` or it will not be picked up by Copilot.
+  copilot-setup-steps:
+    runs-on: ubuntu-latest
+
+    # Set the permissions to the lowest permissions possible needed for your steps.
+    # Copilot will be given its own token for its operations.
+    permissions:
+      # Needed to clone the repository
+      contents: read
+
+    # You can define any steps you want, and they will run before the agent starts.
+    # If you do not check out your code, Copilot will do this for you.
+    steps:
+      - name: Checkout code
+        uses: actions/checkout@v4
+      - name: Set up Rust
+        run: |
+          rustup toolchain install $(awk -F'"' '/channel/{print $2}' rust-toolchain.toml) --profile minimal --no-self-update --component rustfmt,clippy
+      - name: Set up Nextest
+        run: |
+          curl -LsSf https://get.nexte.st/latest/linux | tar zxf - -C ${CARGO_HOME:-~/.cargo}/bin
+      - name: Set up tun device for Linux userland testing
+        run: |
+          sudo ./litebox_platform_linux_userland/scripts/tun-setup.sh