Replaced all errors.New() with core.NewError() and errors.Is() with
core.Is() across error_test.go, error_example_test.go, utils_test.go.
The "errors" stdlib import is now zero across all test files.
Examples teach agents core.NewError() and core.Is() — not errors.New().
Dogfooding: Core's own tests use Core's own primitives.
Co-Authored-By: Virgil <virgil@lethean.io>
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>