Add span origin provenance

[Stephen Belanger (Qard)]

Summary

• Add span origin provenance to Ruby SDK spans.
• Detect environment provenance from explicit config, Braintrust env vars, CI providers, server platforms, and Ruby deployment envs.
• Exclude Braintrust system provenance attributes from AI span filtering.
• Keep config environment helper methods private so they do not become public API surface.

Validation

• mise exec ruby@3.2 -- bundle exec ruby -Ilib:test test/braintrust/config_test.rb test/braintrust/trace/span_filter_test.rb test/braintrust/trace/span_processor_test.rb
• mise exec ruby@3.2 -- bundle exec standardrb lib/braintrust/config.rb lib/braintrust/trace/span_filter.rb lib/braintrust/trace/span_processor.rb test/braintrust/config_test.rb

[David Elner (delner)]

Thanks for putting this together!

My major questions around this revolve around:

1. Wrapper design / method missing. I'd consider this a blocker.
2. Environment reading. I think we could better consolidate/organize this.
3. Public API from_env change. Might be acceptable, just want to clarify on the story of how this overlaps with our state management story.

These things are manageable, but may require some significant refactoring.