clean-slate

Full board-steward teardown for a fresh-user / first-run test. Use when the user wants to wipe ALL board-steward state — kill the board server(s) and ports, remove autostart, purge ~/.board-steward (port registry + .onboarded marker), delete board.json(s), and uninstall the plugin — so a later reinstall starts truly clean and the first-run picker/popup fires. Triggers: "clean slate", "wipe the board", "kill all…