feat: bt topics explore

[max-braintrust]

Summary

When trying to review Topics results, I found my usual workflow was basically:

1. Inspect the topic labels for a particular facet, sorting by volume, token usage, or cost to find the highest impact labels.
2. Drill into representative traces for an interesting label.
3. Open a trace, then return back to the trace list and keep exploring, or return to the label list and keep exploring other labels.

This PR makes that workflow explicit so you can easily drill down from facets to labels to traces and then back out.

Examples

bt topics facets --window 7d

bt topics classifications --facet Task --sort cost --limit 25

bt topics traces --facet Task --topic-id <topic-id> --sort tokens --limit 10

bt topics explore

[github-actions]

Latest downloadable build artifacts for this PR commit cc56e12175ed:

• Workflow run: https://github.com/braintrustdata/bt/actions/runs/30385264583
• Download all artifacts (GitHub CLI): gh run download 30385264583 --repo braintrustdata/bt
• Installers are published from main automatically. To publish one for a PR branch, run release-canary manually via workflow_dispatch.

Available artifact names

• artifacts-build-global
• artifacts-build-local-x86_64-apple-darwin
• artifacts-build-local-x86_64-pc-windows-msvc
• artifacts-build-local-aarch64-pc-windows-msvc
• artifacts-build-local-x86_64-unknown-linux-musl
• artifacts-build-local-aarch64-apple-darwin
• artifacts-build-local-aarch64-unknown-linux-gnu
• artifacts-build-local-x86_64-unknown-linux-gnu
• artifacts-plan-dist-manifest
• cargo-dist-cache