Add comprehensive coverage measurement with SimpleCov

Enhance test coverage measurement capabilities:

Changes:
- Configure SimpleCov with detailed settings in test_helper.rb
  - Add filters to exclude test/, config/, vendor/ directories
  - Add coverage groups (Controllers, Resources, Serializers, etc.)
  - Enable branch coverage tracking (Ruby 2.5+)
  - Configure multi-format output (HTML + Console)
- Clean up .gitignore (remove duplicate coverage entry)
- Update .dockerignore to exclude coverage artifacts
- Add coverage instructions to CLAUDE.md (local & Docker)

Test results with Rails 6.1.7.10:
- Line Coverage: 92.04% (3491/3793 lines)
- Branch Coverage: 85.13% (853/1002 branches)
- All 674 tests passing

Usage:
  COVERAGE=true bundle exec rake test
  docker-compose run -e COVERAGE=true rails-6.1

🤖 Generated with [Claude Code](https://claude.com/claude-code)

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

Author: takaokouji

test/test_helper.rb

@@ -17,6 +17,30 @@
 
 if ENV['COVERAGE']
   SimpleCov.start do
+    add_filter '/test/'
+    add_filter '/config/'
+    add_filter '/vendor/'
+
+    add_group 'Controllers', 'lib/jsonapi/acts_as_resource_controller'
+    add_group 'Resources', 'lib/jsonapi/resource'
+    add_group 'Serializers', 'lib/jsonapi/serializer'
+    add_group 'Processors', 'lib/jsonapi/processor'
+    add_group 'ActiveRelation', 'lib/jsonapi/active_relation'
+    add_group 'Routing', 'lib/jsonapi/routing'
+
+    track_files 'lib/**/*.rb'
+
+    # Optional: Set minimum coverage threshold
+    # minimum_coverage 90
+
+    # Enable branch coverage (requires Ruby 2.5+)
+    enable_coverage :branch if respond_to?(:enable_coverage)
+
+    # Formatting options
+    formatter SimpleCov::Formatter::MultiFormatter.new([
+      SimpleCov::Formatter::HTMLFormatter,
+      SimpleCov::Formatter::SimpleFormatter  # Console output
+    ])
   end
 end