docs: restructure README for users and contributors

[daichunghy]

Summary

• rewrite the README around the maintainer problem and a short first-use path
• move detailed architecture and evidence material behind focused docs links
• make the security boundary and consented pilot path easy to find
• remove grant/application dossier links from the project front page

Verification

• documentation-only change
• npm run verify is unchanged and remains the required project check
• no release, package, or Action behavior changed

This keeps the public surface focused on users, maintainers, and contributors. It does not claim adoption or production readiness.