Update Braintrust tooling guidance for Flue 2 instrumentation API

[astrobot-houston]

Originally proposed by @lforst in #570. Their description is reproduced verbatim below.

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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

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Original implementation from #570 by @lforst