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).
Replace single brackets with double brackets in bash conditional
expressions across 14 files (28 changes). All scripts use bash
shebangs so [[ is safe everywhere (SonarCloud rule shelldre:S7688).
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