Resolve the version source to four outcomes (valid, absent, ambiguous,
broken) in lib/version-source.ts and read that one signal in
gstack-version-bump (new NO_VERSION state; write refuses to invent a
VERSION), gstack-next-version (no_version, no slot), /ship's bump,
CHANGELOG and title steps, /land-and-deploy's drift check and
/document-release. A configured version file that is missing, empty,
unreadable or malformed stops with its path; 0.0.0.0 is never
substituted. docs/skills.md documents .gstack/version-path.
Migrates two tests that pinned the old fabricated-0.0.0.0 behavior.
Co-authored-by: Koda <koda@edgeviewfinance.com.au>
Co-authored-by: Merit Blake <merit@newsprint.com>
