docs: redesign README for developer onboarding

[aakbarpour]

Summary

Rewrites README.md into a concise, developer-first introduction to Flect.

What changed

• Leads with the problem, value proposition, and Codex Skill quick start.
• Adds a Mermaid verification flow, integration comparison, verdict guide, trust/privacy summary, CLI reference, documentation map, status, and attribution.
• Presents RETRACE research uplift with explicit third-party attribution; does not claim those results for Flect.
• Removes the local 20/40-case benchmark narrative, fixture/test-seam details, long internal configuration/protocol material, and maintainer build/test content.
• Keeps the change limited to README.md.

Validation

• cargo fmt --all -- --check
• cargo clippy --workspace --all-targets --all-features -- -D warnings
• cargo test --workspace
• Flect blind verifier + separate reconciliation judge: SAME / SHIP

Closes #55