Decide Draft 2020-12 output contract

[crmne]

Closes #32

Summary

• Make to_json_schema return a pure Draft 2020-12 JSON Schema document
• Add to_json_schema_document as an alias for the pure output
• Keep the old RubyLLM/provider envelope available as to_ruby_llm_schema
• Map schema names to JSON Schema title when no explicit root title exists
• Omit provider-only strict from pure output and stringify keys for JSON compatibility
• Document the migration path

Verification

• bundle exec rake