systemd returns NRestarts=0 for units with LoadState=not-found, making
it indistinguishable from a genuinely loaded unit with zero restarts.
GetNumRestarts now checks LoadState when NRestarts is 0 and returns
ErrValueNotSet for units that don't exist, matching GetMemoryUsage
behavior.
Also adds unit tests for filterErr (all stderr error mapping cases)
and HasValidUnitSuffix (all valid unit types + negative cases).
Updates syncthing test expectation from ErrValueNotSet to nil since
loaded-but-inactive units legitimately have NRestarts=0.
* 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
