[Documentation] Add AGENTS.md for AI coding agents

AGENTS.md

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+# AGENTS.md
+
+This file provides guidance to AI coding agents when working with code in this repository.
+
+## Project Overview
+
+RubyUI is a Ruby gem providing a collection of Phlex-based UI components for Rails applications. It follows a copy-and-paste distribution model (not imported as a runtime dependency). Components use Phlex for rendering, Tailwind CSS for styling (merged via `tailwind_merge`), and Stimulus.js controllers for interactivity.
+
+- **Version:** defined in `lib/ruby_ui.rb`
+- **Ruby requirement:** >= 3.2
+- **Docs:** https://rubyui.com/docs/introduction
+
+## Commands
+
+```bash
+# Run all tests + linter (default rake task)
+bundle exec rake
+
+# Tests only
+bundle exec rake test
+
+# Single test file
+bundle exec rake test TEST=test/ruby_ui/button_test.rb
+
+# Linter (StandardRB)
+bundle exec rake standard
+
+# Auto-fix lint issues
+bundle exec standardrb --fix
+```
+
+## Architecture
+
+### Component Pattern
+
+Every component extends `RubyUI::Base < Phlex::HTML` (`lib/ruby_ui/base.rb`). Base provides:
+- `default_attrs` — override to set default HTML attributes/Tailwind classes
+- `@attrs` — merged user attrs + defaults, with Tailwind classes intelligently merged via `TailwindMerge::Merger`
+- Phlex `mix` helper combines attribute hashes
+
+Typical component structure in `lib/ruby_ui/<component_name>/`:
+- `<component>.rb` — the component class with `view_template` method
+- `<component>_docs.rb` — documentation/usage examples (excluded from tests)
+- `<component>_controller.js` — optional Stimulus controller for interactivity
+
+Components use Phlex DSL for rendering (e.g., `button(**attrs, &)` in `view_template`). Variants are handled through initializer params that map to different Tailwind class sets.
+
+### Test Pattern
+
+Tests use **Minitest** and extend `ComponentTest` (defined in `test/test_helper.rb`). The `phlex` helper renders components to HTML strings for assertion:
+
+```ruby
+class RubyUI::ButtonTest < ComponentTest
+  def test_render_with_all_items
+    output = phlex { RubyUI.Button(variant: :primary) { "Primary" } }
+    assert_match(/Primary/, output)
+  end
+end
+```
+
+Components are invoked via `RubyUI.<ComponentName>()` through Phlex::Kit.
+
+### Generator System
+
+Rails generators in `lib/generators/ruby_ui/` handle installation into consumer apps:
+- `ruby_ui:install` — sets up base component, dependencies, initializer
+- `ruby_ui:component <Name>` — copies a specific component into the app
+- `ruby_ui:component:all` — copies all components
+- `dependencies.yml` — maps each component to its required gems, JS packages, and other components
+
+### JavaScript
+
+Stimulus controllers live alongside their Ruby components. JS dependencies (Chart.js, Embla, Floating UI, etc.) are declared in `package.json` and mapped per-component in `dependencies.yml`.
+
+## CI
+
+GitHub Actions (`.github/workflows/ci.yml`) runs `rake test` and `rake standard` against Ruby 3.3 and 3.4 on every push to main and on PRs.
+
+## PR Template
+
+PRs should be prefixed with a category in brackets (e.g., `[Feature]`, `[Bug Fix]`), reference a related issue, include a description with before/after screenshots if applicable, and provide testing instructions.

CLAUDE.md

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+@AGENTS.md