Refresh project documentation

[richvigorito]

Summary

• rewrite the README as a top-level guide to the project and its major components
• add architecture, gameplay, development, and testing docs under docs/
• rewrite deployment docs around local Compose, staging on the homelab stack, and Fly production
• expand the AI-assisted development write-up to cover later docs and ops work

Testing

• markdown link check for README + docs/*.md