convertToUnixPath() rewrites drive letters to MSYS form (R:\ -> /r/),
which native GnuPG (Gpg4win) cannot resolve. Add investigation.md and a
windows-latest reproducer workflow that installs native GnuPG and expects
the action to fail at GPG signature verification.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>