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
* ci: add test pipeline with codecov, staticcheck, and race detection
- Build/test matrix: Go 1.25 + 1.26
- Race detection enabled for all tests
- Coverage uploaded to Codecov (latest Go only)
- staticcheck lint step
- Go module caching via setup-go
* ci: drop Go 1.25 from matrix (go.mod requires 1.26)
