[codex] raise README standard

[paladini]

Summary

• Reframe the README around a clearer product promise, audience, and adoption path.
• Add top-level navigation, a GitHub-safe Mermaid flow, a stronger capability table, and enterprise-friendly defaults.
• Keep installation, manifest, CLI, GitHub Action, examples, security, and project-status details aligned with the current implementation.

Why

The previous README was accurate but read more like a command reference than the front page of a serious open-source project. This update moves it closer to the pattern used by high-visibility OSS projects: crisp positioning, fast orientation, trustworthy defaults, and a path from local trial to CI adoption.

Validation

• npm test
• npm run build
• npm run lint
• npm run typecheck
• npm run format:check
• git diff --check