Release: prepare 2.0.0

[mike927]

Summary

• fix exact-array and nested key-structure false positives
• require String values for Regexp schemas and support scalar root schemas
• remove Rails and ActiveSupport from runtime dependencies
• strengthen generator discovery coverage and Ruby/Rails CI boundaries
• expand unit coverage, documentation, contribution and security guidance
• remove the private roadmap from version control

Breaking changes

This targets 2.0.0 because stricter array/Regexp matching and removal of implicit Rails runtime dependencies can surface consumer changes.

Verification

• bundle exec rake: 115 examples, 0 failures; 32 files, no RuboCop offenses
• Rails 6.1 appraisal: 115 examples, 0 failures
• Rails 8.1 appraisal: 115 examples, 0 failures
• matcher-only bundle: 112 examples, 0 failures
• bundle-audit: no vulnerabilities
• gem build: rspec-json_api 2.0.0 built successfully
• Rails 8.1 consumer probe: 13 existing examples plus 4 focused 2.0 checks, 0 failures

Review

Independent internal code review completed with no remaining Critical, Important, or Minor findings.