Developer contribution guide for Dev Containers CLI

[brooke-hamilton]

Summary

This PR adds a developer contribution guide at docs/contributing-code.md that covers everything a contributor needs to know to work on the Dev Containers CLI.

What's Included

• Prerequisites: Node.js >= 20, Docker, Git, yarn requirements
• Development Environment Setup: Both dev container (recommended) and local setup options
• Project Structure: Overview of the TypeScript sub-projects and their purposes
• Development Workflow: Build commands, running the CLI locally, testing with Mocha/Chai
• VS Code Integration: Build tasks, debug configurations, and editor settings
• Troubleshooting: Common issues like Docker availability, node-pty build failures, and leftover test containers

Files Changed

• docs/contributing-code.md (new file, 168 lines)

This guide complements the existing CONTRIBUTING.md which focuses on the proposal and specification process, while this new guide focuses on the hands-on development aspects.