docs: update Braintrust tooling guidance

[lforst]

Summary

• update the Braintrust example and blueprint to use the Flue 2 instrumentation API from Braintrust 3.27
• simplify the tracing guide and document current spans, configuration, masking, and runtime behavior
• document Braintrust experiment reporting for vitest-evals

Verification

• pnpm run check:types and pnpm run build in examples/braintrust
• pnpm run build in apps/docs
• pnpm run build in packages/cli
• git diff --check

[astrobot-houston]

Thanks for the contribution! We're closing this PR and moving the conversation to the discussion: #578

We've moved to a model where bugs and feature proposals are discussed in issues/discussions before code review, so the community can help prioritize and shape the work. Your branch is linked from the new thread so the implementation isn't lost — please join us there to continue the conversation.

— astrobot 🤖