feat: package bt for homebrew

[Cedric / ViaDézo1er (viadezo1er)]

Package bt for homebrew, as a third-party package.
bt is not eligible for maintainance by the homebrew team.

TODO

1. Create a braintrustdata/homebrew-bt repo, check we can push directly on main
2. Create a read-write HOMEBREW_TAP_TOKEN token to allow bt's CI to automatically update the packaged version on a bt release.

• GitHub App installation token
• Scope: braintrustdata/homebrew-bt
• Permissions: read and write

3. Cut a bt release to test everything

[github-actions]

Latest downloadable build artifacts for this PR commit 50af462faf81:

• Workflow run: https://github.com/braintrustdata/bt/actions/runs/26546721635
• Download all artifacts (GitHub CLI): gh run download 26546721635 --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