Function check_signing_viability 

pub fn check_signing_viability(project_root: &Path) -> SigningViability

Tag-signing viability check (20d): branches on git config gpg.format since the check is a genuinely different code path per format — a GPG-only check would miss the ssh_askpass failure this project’s own release actually hit (Pattern 4). Fail-soft throughout: an absent tool or unset config degrades to an actionable SigningViability::Unknown, never a crash.