Add return 0 as the last statement in ~46 shell functions across
17 files that previously relied on implicit return codes
(SonarCloud rule shelldre:S7682).
Add strict error handling to all scripts:
- 13 scripts get `set -euo pipefail`
- install-macos-defaults.sh gets `set -uo pipefail` (without -e) because
defaults write commands may fail on newer macOS versions
- install-cargo-packages.sh: also add missing source of shared.sh
Replace pip install --user with uv tool install for CLI tools
(ansible) and uv pip install --system for libraries (libtmux, pynvim).
Drop pipx and pipenv (uv replaces both), delete the external
config/asdf/python-packages file, and rename the script to
install-python-packages to reflect the broader scope.