feat: package bt as npm pkg

[Cedric / ViaDézo1er (viadezo1er)]

Need some help to get an NPM_TOKEN to publish bt as an npm package.

Published name would be braintrust/bt since w have the braintrust org name and bt is already taken.
We might want to reach out to see if it could be possible to get the bt npm pkg since it's dead but that's not a priority.

We could have used https://axodotdev.github.io/cargo-dist/book/installers/npm.html since we're already using cargo-dist but it would make braintrust/bt into an installer instead of really being bt.

[github-actions]

Latest downloadable build artifacts for this PR commit c34ee382d417:

• Workflow run: https://github.com/braintrustdata/bt/actions/runs/26487490458
• Download all artifacts (GitHub CLI): gh run download 26487490458 --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-aarch64-pc-windows-msvc
• ``artifacts-build-local-x86_64-pc-windows-msvc
• ``artifacts-build-local-aarch64-apple-darwin
• ``artifacts-build-local-x86_64-unknown-linux-musl
• ``artifacts-build-local-x86_64-unknown-linux-gnu
• ``artifacts-build-local-aarch64-unknown-linux-gnu
• ``artifacts-plan-dist-manifest
• ``cargo-dist-cache

[Luca Forstner (lforst)]

Thought: I vote for naming the package @braintrust/cli. If the binary in package.json is called bt and they install the package with npm i -g @braintrust/cli, they will still be able to invoke the cli globally with bt.

@braintrust/bt is a bit of a pleonasm. Also we already have places where we refer to bt as "CLI":