Redesign README with improved structure and visual hierarchy (#12)

* Refactor README for open-source release

Replace the internal product-spec README with a professional open-source
README featuring centered logo, CI/coverage/license/tech badges, a clean
feature overview, flow diagram, quick start guide, architecture tree,
configuration reference, and contributing section.

https://claude.ai/code/session_01TfegzxmcjiWN7ovfm9g1A3

* Move product spec from README to dedicated SPEC.md

Extract the authoritative product requirements, page spec, scenario
catalog, system behavior spec, and test coverage sections into SPEC.md.
Update AGENTS.md canonical references to point to the new location.

https://claude.ai/code/session_01TfegzxmcjiWN7ovfm9g1A3

---------

Co-authored-by: Claude <noreply@anthropic.com>

Author: callicles

AGENTS.md

@@ -1,8 +1,8 @@
 # Repository Guidelines
 
 ## Canonical References
-- Product behavior and route/spec details live in `README.md` (`Page Spec`, `Scenario Catalog`, and `System Behavior Spec` sections).
-- Local setup, required environment variables, and command usage live in `README.md` (`Requirements (Local Dev)`, `Dev Setup`, and `Lifecycle Commands` sections).
+- Product behavior and route/spec details live in `SPEC.md` (`Page Spec`, `Scenario Catalog`, and `System Behavior Spec` sections).
+- Local setup, required environment variables, and command usage live in `README.md`.
 
 ## Project Structure & Module Organization
 This repo is a Next.js 16 + TypeScript app-router project.