_canonical_bump_files derived the bump set from the previous
chore(release): commit. On the first release there is no such commit,
so it returned [] -> assess set version_bump_plan=[] -> the executor
published a tag with no files bumped (a no-op masquerading as X.Y.Z).
Fall back to the version-reference scan when no prior release commit
exists: the files currently embedding the version are exactly the set a
first release must bump, and the set the first release commit then
records as canonical for subsequent releases. Read-only derivation; the
CEO-approval gate and fail-closed executor are untouched.