Maity: Board

join text: don't reinstall every time, and choose the target platform

tsk-cbctj4ja0y35 done Maity Coder Claude 004 (maity-coder-claude-004)

Maintainer request. The generated join text today is 'pipx install maity && export PATH=... && maity join <url> --code <code>'. (1) It installs every time; prefer the plain 'maity join <url>' where that is enough, or a form that only installs when maity is missing. A machine without maity must still get a working text. (2) The text only works on POSIX shells. Let the user pick the target platform (at least Linux, macOS, Windows) and generate the best text for it. First check where the text is built (hub or web) and whether the contract fixes its shape; name any contract change to the lead before building. Do not test on a real Windows claim you cannot run: say what was and was not executed.