docs: Add a short Architecture/Modules overview to README

[zhuamber370]

What

Add a short section to README.md that explains the main modules at a glance:

• Backend (FastAPI)
• Frontend (Next.js)
• Governance (changes: dry-run / commit / undo)

Where to start

• Main README: README.md
• Reference docs: backend/README.md, frontend/README.md, openclaw-skill/kms/SKILL.md

Why

README lists scope, but contributors benefit from a 30-second mental model before diving into code.

Acceptance criteria

• New section is 5–10 lines
• Links to backend + frontend READMEs
• Keeps claims verifiable (no marketing fluff)

Verification

• No runtime verification needed (docs-only).