Document the codebase and agent-commerce positioning

[solomon2773]

Summary

• replace the minimal README with a detailed architecture, business-flow, security, testing, and deployment guide
• add a marketing-focused agent-commerce section covering delegated purchasing, ACP, x402/USDC, metered infrastructure, and target use cases
• add scoped AGENTS guidance at the repository, App Router, Prisma, and extension boundaries
• align environment, contribution, architecture, and roadmap documentation with the completed stack

Validation

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

Stacked on #28.