chore: Upgrade to ActiveAdmin 4.0.0.beta20 and simplify test matrix

- Update ActiveAdmin from beta16 to beta20
- Drop support for Rails 6.1, 7.0, 7.1 and ActiveAdmin 2.x/3.x
- Minimum requirements now: Rails 7.2+, Ruby 3.2+, ActiveAdmin 4.x
- Reduce CI jobs from 22+ to 6 (2 gemfiles × 3 Ruby versions)
- Clean up obsolete documentation files

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

docs/setup-activeadmin-gem.md

@@ -0,0 +1,181 @@
+# Setting Up an ActiveAdmin 4 Extension Gem (Combustion)
+
+This guide mirrors the working setup used in this repo: ActiveAdmin 4.0.0.beta20 + Tailwind CSS v4 + Combustion test app.
+
+## 1. Gem Dependencies
+
+```ruby
+# Gemfile
+source "https://rubygems.org"
+gemspec
+
+gem "rails", "~> 7.2"
+gem "activeadmin", "4.0.0.beta20"
+gem "sqlite3"
+gem "puma"
+
+gem "importmap-rails", "~> 2.0"
+gem "propshaft"
+
+gem "tailwindcss-rails", "~> 4.4.0"
+
+group :development, :test do
+  gem "combustion", "~> 1.3"
+  gem "rspec-rails"
+  gem "capybara"
+end
+```
+
+## 2. Combustion Test App
+
+```bash
+bundle exec combust
+```
+
+## 3. config.ru Loading Order
+
+```ruby
+# config.ru
+require "rubygems"
+require "bundler"
+
+Bundler.setup(:default, :development)
+
+require "combustion"
+Combustion.initialize! :active_record, :action_controller, :action_view do
+  config.load_defaults Rails::VERSION::STRING.to_f if Rails::VERSION::MAJOR >= 7
+end
+
+require "importmap-rails"
+require "active_admin"
+require "activeadmin_your_feature"
+
+run Combustion::Application
+```
+
+## 4. Internal App Structure
+
+```bash
+cd spec/internal
+mkdir -p app/admin app/assets/{builds,config,stylesheets} app/javascript
+```
+
+## 5. ActiveAdmin Setup
+
+```ruby
+# spec/internal/config/initializers/active_admin.rb
+ActiveAdmin.setup do |config|
+  config.site_title = "Test App"
+  config.authentication_method = false
+  config.current_user_method = false
+end
+```
+
+## 6. Tailwind Input + Config
+
+```css
+/* spec/internal/app/assets/stylesheets/active_admin.tailwind.css */
+@import "tailwindcss";
+@config "../../../tailwind-active_admin.config.mjs";
+
+/* Import your gem CSS if needed */
+@import "your_gem/css";
+```
+
+```javascript
+// spec/internal/tailwind-active_admin.config.mjs
+import { execSync } from "child_process";
+import activeAdminPlugin from "@activeadmin/activeadmin/plugin";
+
+const activeAdminPath = execSync("bundle show activeadmin", {
+  encoding: "utf-8"
+}).trim();
+
+export default {
+  content: [
+    `${activeAdminPath}/vendor/javascript/flowbite.js`,
+    `${activeAdminPath}/plugin.js`,
+    `${activeAdminPath}/app/views/**/*.{arb,erb,html,rb}`,
+    "./app/admin/**/*.{arb,erb,html,rb}",
+    "./app/views/**/*.{arb,erb,html,rb}",
+    "./app/javascript/**/*.js",
+    "../../app/assets/**/*.{js,css}"
+  ],
+  darkMode: "selector",
+  plugins: [activeAdminPlugin]
+};
+```
+
+## 7. Tailwind Build Tasks
+
+```ruby
+# spec/internal/lib/tasks/active_admin.rake
+namespace :active_admin do
+  desc "Build Active Admin Tailwind stylesheets"
+  task build: :environment do
+    command = [
+      Rails.root.join("bin/tailwindcss").to_s,
+      "-i", Rails.root.join("app/assets/stylesheets/active_admin.tailwind.css").to_s,
+      "-o", Rails.root.join("app/assets/builds/active_admin.css").to_s,
+      "-m"
+    ]
+
+    system(*command, exception: true)
+  end
+end
+```
+
+## 8. Tailwind Binstub
+
+```bash
+bundle binstubs tailwindcss-ruby --force
+```
+
+## 9. JS Entry
+
+```javascript
+// spec/internal/app/javascript/active_admin.js
+import "@activeadmin/activeadmin";
+import "your_gem";
+```
+
+## 10. package.json (internal)
+
+```json
+{
+  "name": "internal",
+  "private": true,
+  "scripts": {
+    "build:js": "node esbuild.config.js",
+    "build:css": "bundle exec rake active_admin:build",
+    "build": "npm run build:js && npm run build:css"
+  },
+  "devDependencies": {
+    "@activeadmin/activeadmin": "^4.0.0-beta20",
+    "esbuild": "^0.24.2"
+  }
+}
+```
+
+## 11. ActiveAdmin 4.0.0.beta20 Notes (from upgrade guide)
+
+**Version Requirements:**
+- Rails 7.2+ (Rails 7.0 and 7.1 are no longer supported)
+- Ruby 3.2+ (Ruby 3.0 and 3.1 are no longer supported)
+
+**Breaking Changes from earlier v4 betas:**
+- `_site_header.html.erb` container class changed from `sticky` to `fixed`.
+- `active_admin.html.erb` adds the `pt-16` utility class.
+- Tailwind v4 requires `@import "tailwindcss"` + `@config`.
+- jQuery and jQuery UI removed; `columns` and `tabs` components removed.
+- Replace `default_main_content` with `render "show_default"`.
+- Replace `as: :datepicker` with `as: :date_picker`.
+- Replace `active_admin_comments` with `active_admin_comments_for(resource)`.
+- Replace `attributes_table` with `attributes_table_for(resource)` in sidebars.
+
+**New in beta20:**
+- Parent menu item linking support
+- Improved comments pagination styling
+- Better vertical spacing for has-many forms
+- Pointer cursor restored on eligible buttons
+- Various accessibility improvements