Implements Linux GPG signing for the release pipeline by adding a `sign-linux.sh` helper, wiring it into the release and test-release workflows, including `.asc` signature files in release collectors, and documenting the full signing workflow in CODE_SIGNING.md with a note in the desktop README.
Fusion-Task-Id: FN-5605
Co-authored-by: Fusion (runfusion.ai) <noreply@runfusion.ai>
Fusion-Task-Id: FN-5605
