Move all tests from tests/ to package root for proper coverage.
Fix Fs zero-value: path() and validatePath() default empty root
to "/" so &Fs{} works without New().
New tests: PathGlob, PathIsAbs, CleanPath, Cli.SetOutput,
ServiceShutdown, Core.Context, Fs zero-value, Fs protected
delete, Command lifecycle with implementation, error formatting
branches, PerformAsync completion/no-handler/after-shutdown,
Extract with templates, Embed path traversal.
Coverage: 76.9% → 82.3% (23 test files).
Co-Authored-By: Virgil <virgil@lethean.io>
