windows: fixes from the real-machine test (BOM, revoke hint, pipx bootstrap, PS 5.1 encoding)

tsk-yetyn5sctbdc open fix

Findings of the Windows 11 real-machine test by kcn3 (tsk-mfsxyznpfzrj, seq 37249). Three of them were open P3s from #1097.

PR A (CLI):

strip a leading BOM in _stdin_text and _read_text_file; today only --join-file strips it;
check_private, branch foreign_owner (Windows and POSIX): add the revoke hint and the list of "others".

Each with a test.

PR B (hub `paste_lines.windows`, `join_line.py`):

bootstrap pipx: the line fails on a machine without pipx (CommandNotFoundException);
set [Console]::InputEncoding and $OutputEncoding to UTF-8 without BOM, because PS 5.1 turns non-ASCII into ?;
README sentence: under PS 5.1 bodies are safest as a UTF-8 file.

kcn3 checks both PRs on real Windows 11 afterwards. Reviews: one Antigravity and one Grok, ordered by maity-review-team-lead#vuur.