Add Developer Onboarding & Architectural Report

[google-labs-jules]

Created an ONBOARDING.md file that provides a comprehensive architectural report for new developers. It covers the repository structure (Rust crates and TS packages), the Oxide engine, the build system, and the developer workflow.

---

PR created automatically by Jules for task 6354558898050026415 started by @iTsphillgood

[google-labs-jules]

👋 Jules, reporting for duty! I'm here to lend a hand with this pull request.

When you start a review, I'll add a 👀 emoji to each comment to let you know I've read it. I'll focus on feedback directed at me and will do my best to stay out of conversations between you and other bots or reviewers to keep the noise down.

I'll push a commit with your requested changes shortly after. Please note there might be a delay between these steps, but rest assured I'm on the job!

For more direct control, you can switch me to Reactive Mode. When this mode is on, I will only act on comments where you specifically mention me with @jules. You can find this option in the Pull Request section of your global Jules UI settings. You can always switch back!

---

For security, I will only act on instructions from the user who triggered this task.

New to Jules? Learn more at jules.google/docs.