docs: polish repository presentation

[pnlabs-dev]

Summary

Polishes the public presentation of proxy-outcome without changing runtime behavior.

• stronger evidence-first positioning at the top of the README;
• consistent PN Labs reliability-toolkit navigation;
• explicit alpha/runtime/local-only status line;
• contribution guidance;
• sanitized bug-report template;
• pull-request checklist that preserves the public/private and security boundaries.

Scope

Documentation and repository-contributor experience only. No classifier or packaging logic is changed.

Safety

No credentials, private endpoints/IPs, production captures, customer data, or proprietary routing/scoring logic are introduced.