docs: align README with public specification

[abdalrahmanattya]

Summary

• Make README the self-contained public entry point.
• Clarify what the service is, what it does, end-to-end GitOps flow, exact local usage, verified release status, safety boundaries, and non-goals.
• Remove internal project/learner/roadmap framing while preserving accurate architecture links and technical details.

Verification

• ./scripts/test-overlays.sh
• ./scripts/check-public-secrets.sh
• xmllint --noout docs/architecture.svg
• README local-link audit
• git diff --check

README.md is the only changed file.