add: buildkite-mcp-server

[sj26]

This adds buildkite-mcp-server as an installable cask:

$ brew install buildkite/buildkite/buildkite-mcp-server
==> Auto-updating Homebrew...
Adjust how often this is run with HOMEBREW_AUTO_UPDATE_SECS or disable with
HOMEBREW_NO_AUTO_UPDATE. Hide these hints with HOMEBREW_NO_ENV_HINTS (see `man brew`).
==> Downloading https://github.com/buildkite/buildkite-mcp-server/releases/download/v0.5.5/buildkite-mcp-server_Darwin_arm64.tar.gz
Already downloaded: /Users/sj26/Library/Caches/Homebrew/downloads/e4ea39ee1b140052feff876f6ae31313b6296b54301e6ee57f615cf850469731--buildkite-mcp-server_Darwin_arm64.tar.gz
==> Installing Cask buildkite-mcp-server
==> Linking Binary 'buildkite-mcp-server' to '/opt/homebrew/bin/buildkite-mcp-server'
🍺  buildkite-mcp-server was successfully installed!

$ buildkite-mcp-server --version
0.5.5

This cask was generated with goreleaser and will be automatically updated based on buildkite/buildkite-mcp-server#87.