filterErr checked 'does not exist' before 'Interactive authentication
required', so when systemd printed both (common for mask/unmask on
non-installed units as a non-root user), the wrong error was returned.
Reorder checks so permission, bus, and masked errors take priority over
existence warnings. Add tests covering mixed-stderr scenarios.
Also:
- CI: install and start nginx so user + root tests pass
- CI: run tests as both user and root for full coverage
- Bump Go 1.26 → 1.26.1
* refactor: clean up unused code, fix typos, improve docs
- Remove unused 'killed' const and 'unitTypes' var (staticcheck U1000)
- Replace regexp with strings.TrimSuffix+switch in isFailed for consistency
- Fix typo: 'programatically' -> 'programmatically'
- Fix typo: 'an an int' -> 'as an int' in README and helpers.go
- Add missing godoc comments on exported helper functions
- Bump minimum Go version from 1.18 to 1.21
* refactor: use unused constants instead of removing them
- Export unitTypes as UnitTypes and add HasValidUnitSuffix helper
- Use killed const (exit code 130) in execute() to detect SIGINT
- Update go.mod to go 1.26
