docs: add layered-rails guidance to README and AGENTS.md

[mroderick]

Summary

Drafts documentation to explain and encourage the use of Layered Rails principles in the codebase, as described in Layered Design for Ruby on Rails Applications by Vladimir Dementyev.

Changes

README.md — new Architectural approach section:

• Describes the four-layer architecture (Presentation, Application, Domain, Infrastructure)
• Maps each layer to key directories in this project
• Contributor guidelines: thin controllers, domain logic in models, no Current in models, extract queries/presenters/form objects

AGENTS.md — new For planning agents block:

• Lists four commands (plan, analyze, review, spec-test) from the layered-rails skill for AI agents working on this codebase

References

• Layered Design for Ruby on Rails Applications (Packt)
• palkan/skills — Claude Code skills for layered design workflows

[olleolleolle]

Good! This adds necessary signposts for further study, and lays out some direct principles.