docs: streamline README and replace broken diagram

[AetherAI3]

What changed

• Rebuilt the README around a clear reader path: value, quick start, architecture flow, strategy library, boundaries, and contribution links.
• Replaced the broken Mermaid block with a committed SVG, so the visual displays without a Mermaid parser.
• Made the strategy library a first-class community entry point while moving exhaustive detail into linked references.

Why

The default-branch README had a broken diagram and too much detail competing on the landing page. This is a direct, self-contained cleanup of that experience.

Validation

• README source regression and Python compilation checks passed.
• README local links, SVG XML, and git diff --check passed.

Supersedes draft PR #3.