docs: add production readiness guide to README

[CoreyLeath-code]

Summary

Adds a standardized production-readiness guide to the README for HelixAgent.

Included

• working CI/license status links;
• architecture flowchart;
• clean-checkout quickstart and validation commands;
• research-style metrics and benchmark evidence requirements;
• extended production-readiness Q&A;
• explicit distinction between portfolio engineering evidence and production authorization.

This PR is documentation-only and pairs with the repository's production-readiness audit issue.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch docs/portfolio-readme-production-helixagent

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