Move runner.go, runner_windows.go, runner_other.go, runner_test.go from
src/core to src/runner (package runner). Split helpers into focused files:
invocation*.go, exitcodes.go, logfile.go, cleanup.go. Export LogArguments
and SuccessExitCodesText for use by scheduler's runningOutput helper.
Update src/core/scheduler.go and src/gui/app.go to import src/runner.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
mixeme