docs(readme): overhaul project overview

[connorcarpenter15]

Summary

• redesign the README header with project badges, quick links, and a table of contents
• describe the current OpenEngine architecture, capabilities, RPC surface, generation stream, error model, and schema compatibility contract
• add validated setup and schema-checking commands, repository navigation, project status, contribution, security, license, and ecosystem links
• preserve the experimental pre-adoption status without presenting the API as stable or implemented today

Why

The previous README predates the latest generation, discovery, error, and lifecycle contract changes. This update makes the repository easier to understand and navigate while giving OpenEngine a polished project landing page.

Validation

• GitHub GFM rendering API
• local link and asset target checks
• buf build
• buf lint
• buf breaking --against '.git#branch=main,subdir=proto'
• git diff --check