Windows has no POSIX execute bit; Node reports file modes from the
read-only attribute alone, so every host file, including PE images,
reported `executable: false` while the same occurrence reported
`artifact_kind: executable`. Derive the host executable marking once:
POSIX permission bits stay authoritative, and Windows marks regular
files with the CreateProcess/cmd launchable extensions.
Fixes#1819
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>
