The macOS head is AppKit: a menu bar item and a window, drawn by a Swift library the tool carries for both Intel and Apple silicon. There is no app bundle; the tool is a plain executable, like every dotnet tool. The Dock icon may flash briefly at start, which is the price of that, and failure notifications go through osascript because Notification Center only takes them from a bundle.
Tokens are stored in the login Keychain as generic passwords under the service name BuildMonitor, through the security command. macOS may ask once to allow access after an update, because the executable that asks has moved.
A launch agent, ~/Library/LaunchAgents/com.simoncropp.buildmonitor.plist, loaded with launchctl bootstrap. It starts the tray once at login and does not restart it after Exit.