Add RUBYLLM_LOG_FILE environment support

[Gujiassh]

Summary

• let RubyLLM::Configuration.log_file default to ENV['RUBYLLM_LOG_FILE'] before falling back to $stdout
• keep the feature scoped to configuration + logger initialization without touching the logging pipeline itself
• add a focused logger spec covering the env-var default path

Validation

• bundle exec rspec spec/ruby_llm_spec.rb
• bundle exec rubocop lib/ruby_llm/configuration.rb spec/ruby_llm_spec.rb