A failed venv creation (e.g. the Debian/Ubuntu python3-venv package
missing) left the .venv directory behind; the next run saw it, skipped
creation and failed to source/activate a venv without activate/pip.
setup.sh now validates bin/activate, bin/python and bin/pip, removes an
incomplete directory and creates it again; a creation failure removes the
partial directory and names the venv package. setup.bat does the same
with Scripts\activate.bat/python.exe/pip.exe and !errorlevel!.
READMEs (prerequisites + scripts table), CHANGELOG (folded into the
unreleased 3.38.1) and AGENTS updated.
Tests: tests/test_setup_scripts.py (5: the shell/bat source pins and
bash -n). 1088 Python + 1075 frontend.
