Document the codebase architecture and contributor workflows

[solomon2773]

Summary

• replace the minimal README with a detailed architecture, business-flow, security, testing, and deployment guide
• add scoped AGENTS guidance at the repository, App Router, Prisma, and extension boundaries
• align environment, contribution, architecture, and roadmap documentation with the completed stack
• document remaining operational follow-ups and the future anonymous autonomous account roadmap item

Validation

• documentation links and structure reviewed against the final implementation
• repository whitespace and prohibited-attribution scans pass

Stacked on #19.