Redraw the README architecture diagram

[SaladDay]

Summary

• Replace the Mermaid rendering in docs/assets/architecture.svg with the deployment and protocol-boundary diagram: control server, microsandbox compute servers, E2B, self-hosted machines, independent workspace storage and external services, with each protocol labeled on its connection. The colors follow the README banner.
• Add scripts/generate-architecture-diagram.py, which owns the layout and colors. It embeds IBM Plex Sans from Web's @fontsource dependency so GitHub and the website render the same text metrics, uses only the Python standard library, and fails on text that overflows its box or lacks a glyph.
• Record the generator beside the other README assets in CONTRIBUTING's Branding section.
• Drop the website's white frame around the image; the diagram carries its own background.

Regenerate

npm ci --prefix apps/web   # once, for the embedded font
python3 scripts/generate-architecture-diagram.py

Checks

• The generator's output is deterministic (two runs, same SHA-256), and it parses as XML.
• Rendered in Chromium at 1560 px and README width (880 px); the result matches the approved style candidate pixel for pixel outside the legend row, whose spacing no longer depends on a fallback font.
• python3 scripts/check-names.py passes.

---

^{Need help on this PR? Tag @codesmith-bot with what you need.}